U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5218

Prepared in cooperation with the

Bureau of Reclamation and the

Colorado River Salinity Control Forum

Characterization of Hydrology and Salinity in the Dolores 

Project Area, McElmo Creek Region, Southwest Colorado, 

Water Years 1978–2006

Characterization of Hydrology and Salinity 

in the Dolores Project Area, McElmo Creek 

Region, Southwest Colorado, Water Years 

1978–2006

By Rodney J. Richards and Kenneth J. Leib

Prepared in cooperation with the Bureau of Reclamation and the 

Colorado River Salinity Control Forum

Scientific Investigations Report 2010–5218

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Richards, R.J., and Leib, K.J., 2011, Characterization of hydrology and salinity in the Dolores project area, McElmo 

Creek Region, southwest Colorado, 1978–2006: U.S. Geological Survey Scientific Investigations Report 2010–5218, 

38 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................3

Description of Study Area ...................................................................................................................6

The Dolores Project.....................................................................................................................................10

Methods of Analysis....................................................................................................................................10

Site Selection ......................................................................................................................................10

Data Analysis.......................................................................................................................................13

Multivariate Regression Models.............................................................................................13

Salinity-Load Calculations........................................................................................................13

Trend Analysis ............................................................................................................................15

Characterization of Hydrology and Salinity in McElmo Creek .............................................................17

Hydrology .............................................................................................................................................17

Salinity ..................................................................................................................................................20

Effect of Irrigation-Delivery and Salinity-Control Work by the Dolores Project.......................20

Flow-Adjusted SalinityTrends ..................................................................................................23

Trend Analysis in the Mud Creek Subbasin...........................................................................27

Natural Salinity in McElmo Creek Basin.........................................................................................28

Summary........................................................................................................................................................29

Acknowledgments.......................................................................................................................................31

References Cited..........................................................................................................................................31

Appendix: S-LOADEST Salinity Load Output Tables...............................................................................33

Figures

1–5. Maps showing:

 1. Location of McElmo Creek basin and location of streamgage sites used in 

the analysis of salinity load..................................................................................................2

 2. Configuration of canals before construction of McPhee Reservoir, 

southwest Colorado ..............................................................................................................4

 3. Configuration of canals after construction of McPhee Reservoir, 

southwest Colorado ..............................................................................................................5

 4. Irrigated lands in the year 2000 and canal configuration after completion of 

McPhee Reservoir, southwest Colorado...........................................................................7

 5. Surficial exposures of geologic units in the McElmo Creek watershed and 

surrounding area ...................................................................................................................8

6. Timeline of events related to the completion of the Dolores Project from 

1965 through 2004................................................................................................................................11

7. (A) Daily streamflow for site 1, McElmo Creek near Colorado-Utah state line, 

from March 1951 to December 2005. (B) Streamflow in acre-feet per year at 

site 1, McElmo Creek near Colorado-Utah state line, for water years 1952–2005...........19

8. Boxplots showing distribution of daily mean salinity concentrations at various 

sites in McElmo Creek basin, southwest Colorado...............................................................22
iv

9. Pipe diagram comparing non-flow-adjusted salinity load in tons from water 

years 1978–1984 (preperiod) to salinity load in tons from water years 

2000–2006 (postperiod) at selected sites in the McElmo Creek basin 

area, southwest Colorado .........................................................................................................24

10. Partial residual plot of salinity concentration with LOWESS curve at 

(A ) McElmo Creek near Colorado–Utah State line (site 1) and 

(B ) Dolores River at Dolores, Colorado (site 17)....................................................................25

11. Partial residual plot of salinity concentrations with LOWESS curve of 

Mud Creek at State Highway 32 near Cortez, Colorado (site 6)..........................................26

Tables

1. Initial and updated estimates of salinity loading as a result of the Dolores 

Project, southwest Colorado ....................................................................................................12

2. Water-quality sites used in the analysis of salinity load in the McElmo Creek 

basin, southwest Colorado........................................................................................................12

3. Summary of equation 2 used at selected sites in the McElmo Creek region, 

southwest Colorado ...................................................................................................................15

4. Summary of equation forms used to estimate salinity load at the sampling sites ..........17

5. Summary of regression model coefficients and diagnostics for three 

sampling sites..............................................................................................................................17

6. Summary of daily streamflow data for selected steamgaging stations in or 

near the McElmo Creek basin, southwest Colorado ............................................................18

7. Summary statistics for daily mean salinity concentration for selected sites 

in the McElmo Creek region from water years 1978–1984 (preperiod) and 

water years 2000–2006 (postperiod)........................................................................................21

8. Salinity balance with comparisons of preperiod and postperiod salinity loads 

for inflow and outflow sites.......................................................................................................23

9. Salinity-load estimates and flow-adjusted trend-analysis summary for water 

years 1978–2006 ..........................................................................................................................26

10. Salinity load estimates and flow-adjusted trend analysis summary for water 

years 1989–2006 ..........................................................................................................................27

11. Summary of salinity-load reductions and trend analysis at site 1 for water 

years 1978–2006 ..........................................................................................................................28

12. Historical salinity load summary for McElmo Creek basin, southwest Colorado............29

13. S-LOADEST salinity load output1 for site 1 from water year 1978 to water year 2006....33

14. S-LOADEST salinity load output1 for site 1 from water year 1989 to water year 2006....34

15. S-LOADEST salinity load output1 for site 6 from water year 1982 to water year 2006....35

16. S-LOADEST salinity load output1 for site 6 from water year 1990 to water year 2006....36

17. S-LOADEST salinity load output1 for site 17 from water year 1978 to water year 2006..37

18. S-LOADEST salinity load output1 for site 17 from water year 1989 to water year 2006..38
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, nautical (nmi) 1.852 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

acre 4,047 square meter (m2

)

acre 0.004047 square kilometer (km2

)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Flow rate

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3

/yr)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

ton per day (ton/d) 0.9072 metric ton per day

ton per year (ton/yr) 0.9072 metric ton per year

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Water year is defined in this report as the 12-month period October 1 through September 30, 

designated by the calendar year in which it ends.
vi

Acronyms

USGS U.S. Geological Survey

USBR Bureau of Reclamation

NRCS Natural Resources Conservation Service

CDWR Colorado Division of Water Resources

CRSCF Colorado River Salinity Control Forum
Characterization of Hydrology and Salinity in the Dolores 

Project Area, McElmo Creek Region, Southwest Colorado, 

Water Years 1978–2006

By Rodney J. Richards and Kenneth J. Leib

Abstract

Increasing salinity loading in the Colorado River has 

become a major concern for agricultural and municipal water 

supplies. The Colorado Salinity Control Act was implemented 

in 1974 to protect and enhance the quality of water in the 

Colorado River Basin. The U.S. Geological Survey, in cooperation with the Bureau of Reclamation and the Colorado 

River Salinity Control Forum, summarized salinity reductions 

in the McElmo Creek basin in southwest Colorado as a result 

of salinity-control modifications and flow-regime changes that 

result from the Dolores Project, which consists of the construction of McPhee reservoir on the Dolores River and salinity control modifications along the irrigation water delivery 

system. 

Flow-adjusted salinity trends using S-LOADEST estimations for a streamgage on McElmo Creek (site 1), that represents outflow from the basin, indicates a decrease in salinity 

load by 39,800 tons from water year 1978 through water year 

2006, which is an average decrease of 1,370 tons per year for 

the 29-year period. Annual-load calculations for a streamgage 

on Mud Creek (site 6), that represents outflow from a tributary 

basin, indicate a decrease of 7,300 tons from water year 1982 

through water year 2006, which is an average decrease of 292 

tons per year for the 25-year period. The streamgage Dolores River at Dolores, CO (site 17) was chosen to represent a 

background site that is not affected by the Dolores Project. 

Annual load calculations for site 17 estimated a decrease of 

about 8,600 tons from water year 1978 through water year 

2006, which is an average decrease of 297 tons per year for 

the 29-year period. The trend in salinity load at site 17 was 

considered to be representative of a natural trend in the region. 

Typically, salinity concentrations at outflow sites 

decreased from the pre-Dolores Project period (water years 

1978–1984) to the post-Dolores Project period (water years 

2000–2006). The median salinity concentration for site 1 

(main basin outflow) decreased from 2,210 milligrams per liter 

per day in the preperiod to 2,110 milligrams per liter per day 

in the postperiod. The median salinity concentration for site 6 

(tributary outflow) increased from 3,370 milligrams per liter 

per day in the preperiod to 3,710 milligrams per liter per day 

in the postperiod. Salinity concentrations typically increased 

at inflow sites from the preperiod to the postperiod. Salinity 

concentrations increased from 178 milligrams per liter per day 

during the preperiod at Main Canal #1 (site 16) to 

227 milligrams per liter per day during the postperiod at the 

Dolores Tunnel Outlet near Dolores, CO (site 15). 

Calculation of the historical flow regime in McElmo 

Creek was done using a water-budget analysis of the basin. 

During water years 2000–2006, an estimated 845,000 acrefeet of water was consumed by crops and did not return to 

the creek as streamflow. The remaining 76,000 acre-feet, 

or 10,900 acre-feet per year for the 7-year postperiod, was 

assumed to represent a historical flow condition. The historical 

flow of 10,900 acre-feet per year is equivalent to 15.1 cubic 

feet per second.

Average total dissolved solids concentrations for water in 

each type of sedimentary rock were used to estimate natural 

salinity loads. Most surface-water sites used to fit the criteria 

needed to achieve a natural TDS concentration were springs. 

An average spring TDS value for sandstones geology in the 

basin was 350 milligrams per liter, and the average value for 

Mancos Shale geology was 4,000 milligrams per liter. The 

natural salinity loads in McElmo Creek were estimated to be 

29,100 tons per year, which is 43 percent of the salinity load 

that was calculated for the postperiod.

Introduction

The Colorado Salinity Control Act, Public Law 93–320, 

June 24, 1974, was enacted to enhance and protect the quality 

of water in the Colorado River. Title II of the Colorado Salinity Control Act authorized the investigation and implementation of control measures for selected salinity-control units, 

including the McElmo Creek basin in Montezuma County, 

southwestern Colorado (fig. 1) (Bureau of Reclamation, 2003). 

Salinity is a concern in the McElmo Creek region of the Upper 

Colorado River Basin. The development of McPhee Reservoir, 

which is fed by the Dolores River, as part of the Dolores Project in the early 1980s provided additional irrigation water for 
2 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

!( Bureau of Reclamation streamgage site

!( Colorado Division of Water Resources streamgage site

!( U.S. Geological Survey streamgage site

Urban area

McElmo Creek basin boundary

EXPLANATION

1 Site number

!

!

!(

!(

!(!( !(

!(

!(

!( !(

!( !(

!(!(

!(

!(

!(

!(

109°00'W

37°30'N

37°15'N

37°00'N

108°30'W

LA PLATA

MONTEZUMA

MONTROSE

DOLORES

SAN MIGUEL

OURAY

SAN JUAN

Dolores

River

Mancos River

SanJuan

River

Dolores

Cortez

Enlarged 

 area

COLORADO

UTAH

ARIZONA NEW MEXICO

McPhee

Reservoir

0 5 10 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data 1:400,000

Universal Transverse Mercator projection Zone 13 

North American Datum of 1983 (NAD 83)

Navajo Wash

Alkali Canyon

Mud Creek

Cross Canyon

Hartman Draw

Beaver Creek

Dawson Draw

Lost Canyon Creek

Narraguinnop Canyon

Sand Canyon

House Creek

Yellow Jacket Canyon

Simon Draw

Rock Creek

Cash Draw

McElmo Creek

Sandstone Canyon

Dolores River

Trail Canyon

Mancos River

Dolores River

Yellow Jacket Canyon

UV160

UV145

UV184

UV041 UV491

UV160

UV

Cortez

Dolores

1 2

3

4 5

6 7

8 9

10

11 12

13

14

15

16

17

18

San Juan River

McPhee Reservoir

Summit

Reservoir

Narraguinnep 

Reservoir

Figure 1. Location of McElmo Creek basin and location of streamgage sites used in the analysis of salinity load.
Introduction 3

the McElmo Creek basin and lengthened the irrigation season, 

which potentially allowed for increases in salinity to McElmo 

Creek and its tributaries (Bureau of Reclamation, 1988).

The term salinity is defined as a concentration of dissolved mineral salts and solids in water (Hem, 1959). Salinity 

is typically used synonymously with the terms salt or total dissolved solids (TDS). However, in this report, salinity refers to 

total dissolved solids in streams, whereas the term salt is used 

to describe mineral salts that have not been moved or transported to another location and remain in situ. 

McElmo Creek basin has been identified as a major 

contributor of salinity from both irrigation and natural sources 

(fig. 1). The Bureau of Reclamation (USBR) estimates that 

47 percent of the salinity load in the Colorado River Basin is 

derived from natural sources including geological formations, 

saline springs, and surface runoff (Bureau of Reclamation, 

2003). Approximately 37 percent of the salinity load in the 

basin results from irrigation, and the remaining 16 percent 

results from reservoir-storage effects and municipal and industrial practices. Evaporation of exposed water and evapotranspiration by plants can result in the formation of mineral and 

salt deposits, which become dissolved as the deposits come 

in contact with water. Coastal and marine sedimentary rocks, 

such as those that occur in the study area, are substantial 

sources of mineral and salt deposits due to the evapoconcentration of saline waters during the time of deposition. Irrigation water can dissolve near-surface salt deposits through 

canal seepage and percolation beyond the lower limits of the 

root zone into groundwater (deep percolation). The concentrated base flow returns to the stream either as surface water or 

groundwater, increasing the salinity load in the Colorado River 

and tributaries to the Colorado River (Iorns and others, 1965). 

Salinity-control projects such as irrigation and waterdelivery system improvements have been implemented in the 

McElmo Creek basin to reduce the overall salinity load to 

McElmo Creek. Off-farm improvements typically consist of 

canal improvements such as realignments, lining, and piping. 

On-farm improvements typically consist of sprinkler-system 

and irrigation-system upgrades, including the transitional 

delivery system to the field. Costs associated with reducing 

salinity load in the Lower Colorado River rise as salinity concentration increases; therefore, understanding and managing 

salinity sources to the Upper Colorado River Basin potentially 

help reduce future water-treatment costs in the Lower Colorado River Basin (Bureau of Reclamation, 1988). The Dolores 

Project, located in the McElmo Creek salinity-control unit, 

was developed to help support the agricultural industry that 

farms about 62,000 acres of irrigated land in the area. The 

value of crops produced on the irrigated land in 1992 was 

estimated to be about 11 million dollars (Bureau of Reclamation, 2008). To offset the potential increase of salinity and to 

maintain crop productivity, salinity-control features were completed as part of the Dolores Project. These features included 

the reduction of canal seepage by lining, piping, rerouting 

canals, and combining smaller canals into a single larger 

canal. These features raised the efficiency of irrigation-water 

applications in most areas by pressurizing water for sprinklerirrigation systems (Bureau of Reclamation, 1988).

The USBR created a water-quality monitoring network 

before construction of McPhee Reservoir and implementation of salinity-control projects. Data from the network were 

used to estimate salinity loads in the McElmo Creek basin 

before implementation of the Dolores Project. The location of 

selected sites in the network in place before implementation of 

the Dolores Project, along with canals and related structures, 

is shown in figure 2. Samples collected at selected sites before 

implementation of the Dolores Project characterize salinity 

levels that existed when irrigation water from the Dolores 

River was delivered through a headgate at site 16 near the 

current location of McPhee Reservoir (fig. 2). The irrigationdelivery system before implementation of the Dolores Project 

was more prone to water shortages and short growing seasons 

than the new system with McPhee Reservoir. Lack of irrigation water generally has not been an issue since completion 

of the Dolores Project because of increased water-storage 

capabilities and improved water-delivery methods (Bureau of 

Reclamation, 1988). 

Interest in understanding water and salinity budgets and 

the effectiveness of salinity-control efforts since the construction of McPhee Reservoir resulted in a network of waterquality monitoring sites, predominantly operated by the U.S. 

Geological Survey (USGS) and Colorado Division of Water 

Resources (CDWR). The location of selected sites in the 

network in place after the completion of the Dolores Project, 

along with canals and related structures, is shown in figure 3. 

Comparison of the salinity loads exiting the basin during water 

years 1978-1984 and water years 2000-2006 gives insight 

as to the effectiveness of salinity-control features that were 

implemented and whether additional controls are needed. 

Continued interest in determining the effectiveness of 

salinity-control measures within the McElmo Creek basin 

prompted a desire for further analysis. As a result, the USGS, 

in cooperation with the Colorado River Salinity Control 

Forum (CRSCF) and the USBR, developed a study to characterize the hydrology and salinity in the McElmo Creek basin 

and describe the effects of salinity control efforts by comparing salinity loads before construction of the Dolores Project 

and after the completion of salinity-control measures associated with the Dolores Project. 

Purpose and Scope

This report characterizes the hydrology and salinity in 

the McElmo Creek region before and after completion of the 

Dolores Project in southwest Colorado. Based on the available 

water-quality data collected by the USGS, USBR, and CDWR, 

this report (1) describes the hydrology, salinity concentrations, 

and salinity loads of the McElmo Creek basin for the water 

years 1978–1984 (October 1977–September 1984) and water 

years 2000–2006 (October 1999–September 2006); (2) compares initial estimates of increases and decreases in salinity 
4 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Canal

! !

Future location of McPhee Reservoir

!( Bureau of Reclamation streamgage site

!( Colorado Division of Water Resources streamgage site

!( U.S. Geological Survey streamgage site

Urban area

McElmo Creek basin boundary

EXPLANATION

1 Site number

!(

!(

!(!(

!(

!( !(

!(

!(

!(

!(

!(

!(

!(

!(!(

!(

!(!(

!(

!(

UV160

UV145

UV184

UV041

UV491

UV160

UV491

Cortez

Dolores

1

2

3

5

6 7

8 9

10

13

14

16 17

18

12

Lower Hermana Lateral

Highline Ditch

Rocky Ford Ditch

East Lateral

West Lateral

U-Lateral

Lone Pine Lateral

Main Canal #2

Main Canal #1

MESA VERDE NATIONAL PARK

McElmo Creek

Alkali Canyon

Cross Canyon

Hartman Draw

Dolores River

Navajo Wash

Mud Creek

Beaver Creek

Dawson Draw

Sand Canyon

Yellow Jacket Canyon

Simon Draw

Trail Canyon

Rock Creek

Cash Draw

Yellow Jacket Canyon

Sandstone Canyon

Trail Canyon

House Creek

Dolores River

Narraguinnep 

Reservoir

Summit

Reservoir

MONTEZUMA

DOLORES

109°00'W 108°30'W

37°30'N

37°15'N

0 5 10 MILES

0 7 14 KILOMETERS

Base from U.S. Geological Survey digital data 1:300,000

Universal Transverse Mercator projection Zone 13 

North American Datum of 1983 (NAD 83)

Figure 2. Configuration of canals before construction of McPhee Reservoir, southwest Colorado.
Introduction 5

109°00'W 108°30'W

37°30'N

37°15'N

0 5 10 MILES

0 7 14 KILOMETERS

Base from U.S. Geological Survey digital data 1:300,000

Universal Transverse Mercator projection Zone 13 

North American Datum of 1983 (NAD 83)

Canal

! !

!( Bureau of Reclamation streamgage site

!( Colorado Division of Water Resources streamgage site

!( U.S. Geological Survey streamgage site

Urban area

McElmo Creek basin boundary

EXPLANATION

1 Site number

McPhee Reservoir

Narraguinnep 

Reservoir

Summit

Reservoir

!(

!(

!( !(!(

!( !(

!(

!(

!(

!(

!(

!(

!(

!( !(

!(!(

!(

!(

!(

UV160

UV145

UV184

UV041

UV491

UV160

UV491

Cortez

Dolores

1

2

3

4 5

6 7

8 9

10

13 11

15

17

18

12

Towaoc Canal

Hermana Lateral

Main Aqueduct

West 1 Upper Arickere

Lone Pine Lateral

U-Lateral

Dove Creek Canal

MESA VERDE NATIONAL PARK

McElmo Creek

Alkali Canyon

Cross Canyon

Hartman Draw

Dolores River

Navajo Wash

Mud Creek

Beaver Creek

Dawson Draw

Sand Canyon

Yellow Jacket Canyon

Simon Draw

Cash Draw

Trail Canyon

Yellow Jacket Canyon

Rock Creek

Sandstone Canyon

Cash Draw

Trail Canyon

House Creek

Dolores River

MONTEZUMA

DOLORES

Narraguinnep 

Reservoir

Figure 3. Configuration of canals after construction of McPhee Reservoir, southwest Colorado.
6 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

load for the Dolores Project to measured or calculated salinity 

loads in the McElmo Creek basin; (3) creates a timeline that 

relates the Dolores Project activities to changes in streamflow 

and salinity concentrations and loads in McElmo Creek; and 

(4) characterizes natural salinity loads within McElmo Creek. 

Water-quantity and water-quality data from 18 surfacewater sites within and near the McElmo Creek basin were 

analyzed to characterize salinity concentrations and loads for 

water years 1978–1984 and water years 2000–2006. The date 

ranges represent the periods before implementation of the 

Dolores Project (referred to herein as the preperiod) and after 

the completion of the Dolores Project (referred to herein as the 

postperiod). Sites were selected on the basis of completeness 

of the water-quality record for the selected time period and 

location within the basin. 

Flow-adjusted salinity trends over two time periods were 

used to characterize the magnitude and direction of salinity 

trend. Water years 1978–1984 were used to characterize the 

trend for the entire study period and include any changes in 

the McElmo Creek basin that resulted in changes in salinity; 

water years 1989–2006 were used to indicate the magnitude 

of the salinity trend associated with the most active salinity 

control efforts. A water year is defined as the 12-month period 

October 1 through September 30, designated by the calendar 

year in which it ends.

Description of Study Area

The McElmo Creek region of the Upper Colorado River 

Basin is located in southwest Colorado. The creek flows westward from Colorado into Utah, where it joins the San Juan 

River. In Colorado, McElmo Creek lies within Montezuma 

County between the Dolores River and the San Juan River 

(fig. 1). The two largest urban centers in the region are Cortez, 

Colorado, and Dolores, Colorado. Cortez is approximately 

7 miles south of McPhee Reservoir, along the northern bank 

of McElmo Creek, 24 miles east from the Utah – Colorado 

State line. Dolores is approximately 7 miles northwest of the 

headwaters of McElmo Creek and east of McPhee Reservoir. 

The land-surface elevation at Cortez is about 6,200 feet and at 

Dolores is about 6,940 feet (U.S. Geological Survey Geographic Names Information System, 2008). 

A substantial part of the land use in the region is irrigated 

agriculture (fig. 4). The agricultural lands around McElmo 

Creek are irrigated from a system of canals that draw water 

from McPhee Reservoir, Narraguinnep Reservoir, and Summit 

Reservoir. Residential and urban land uses are increasing as 

population increases in the region. The population of Cortez 

increased from 7,977 in 2000 to 8,531 in 2007, and the population of Dolores increased from 857 in 2000 to 905 in 2007. 

The population of Montezuma County increased from 23,830 

in 2000 to 25,221 in 2007 (U.S. Census Bureau, 2008). The 

increase in population has placed increased demand on the 

available water supplies. 

The geology of the study area is influenced by the San 

Juan Mountains to the east and the Monument uplift in southeast Utah. The primary geologic units at or near the surface in 

the study area are the Dakota Sandstone, Burro Canyon Formation, and Mancos Shale of Cretaceous age (fig. 5) (Tweto, 

1979). Quaternary eolian deposits are present in the northwest 

part of the study area and are along some stream channels. 

Jurassic-age Morrison Formation and Entrada Sandstone crop 

out along the western reaches of McElmo Creek. 

Sandstone that was deposited in or near saline environments or has been inundated by saltwater tends to have high 

concentrations of salts containing sodium, sulfate, chloride, 

magnesium, and calcium ions. As freshwater moves through 

porous sandstone, it mobilizes salts and transports them to 

streams and rivers. A similar transport of salt ions occurs in 

shale that was deposited in saline environments. Although not 

as permeable as sandstone, shale is highly porous and water 

that comes in contact with it or infiltrates it tends to pick up 

large amounts of dissolved solids (Hem, 1985). 

Mancos Shale can be a substantial contributor of dissolved solids to water systems. Deposition in a marine setting 

increases the abundance of chloride (Cl2-), sodium (Na1+), 

magnesium (Mg2+), sulfate (SO4

2-), calcium (Ca2+), potassium 

(K1+), carbonic acid (HCO3

1-), and carbonate (CO32- )(Garrels 

and Thompson, 1962). Shale deposits tend to contribute more 

soluble ions than associated sandstone marine deposits and 

typically present a higher salinity hazard (Garrels and Thompson, 1962). Mancos Shale contains appreciable amounts of 

gypsum, which is the predominant soluble mineral in the 

shale, but substantial amounts of sodium and magnesium 

hydrate sulfates also are present. Salt crusts (also known as 

effervescent salts and evaporative facies) are important contributors of salts in precipitation events. The crusts are formed 

from the evapoconcentration of dissolved solids after transport 

to the surface. Crusts develop over Mancos Shale because 

shale bedrock has low permeability and overland flow does 

not readily percolate into the soil, which allows for evaporation of water and chemical precipitation of dissolved solids. 

Alluvium, which has been transported and deposited in 

a fluvial system, has most of its readily soluble ions removed 

and does not contribute dissolved constituents as much as 

bedrock shale deposits. Soils derived from shale typically have 

salt concentrations that are 10 percent higher than alluvium 

(Laronne, 1977).

The climate in the study area is semiarid, with some 

variation due to the topography and proximity of the San Juan 

Mountains. Based on records for 1911–2007, in Cortez, Colo., 

July is the hottest month, with an average maximum temperature of 88.7 degees Fahrenheit, and January is the coldest 

month, with an average minimum temperature of 13.2 degrees 

Fahrenheit. The annual average total precipitation for Cortez 

during the same time period is 13.12 inches, with an average 

annual snowfall of 35.2 inches. In comparison, Dolores, Colo., 

which is slightly outside the McElmo Creek basin, received an 

average of 18.62 inches of precipitation annually from 1908 

to 2004 and had an average annual snowfall of 65.2 inches 
Introduction 7

Irrigated land in 2000 (Techni Graphics System, Inc., 2004)

109°00'W 108°30'W

37°30'N

37°15'N

0 5 10 MILES

0 7 14 KILOMETERS

Base from U.S. Geological Survey digital data 1:300,000

Universal Transverse Mercator projection Zone 13 

North American Datum of 1983 (NAD 83)

Canal

! !

!( Bureau of Reclamation streamgage site

!( Colorado Division of Water Resources streamgage site

!( U.S. Geological Survey streamgage site

Urban area

McElmo Creek basin boundary

EXPLANATION

1 Site number

McPhee Reservoir

Summit

Reservoir

!(

!(

!( !(!(

!( !(

!(

!(

!(

!(

!(

!(

!(

!( !(

!(!(

!(

!(

!(

VU160

VU145

VU184

VU041

VU491

VU160

VU491

Cortez

Dolores

1

2

3

4 5

6

7

8 9

10

13 11

15

17

18

12

Main Aqueduct

West 1 Upper Arickere

Dove Creek Canal

MESA VERDE NATIONAL PARK

McElmo Creek

Alkali Canyon

Hartman Draw

Simon Draw

Cash Draw

Lone Pine Lateral

Narraguinnep 

Reservoir

U-Lateral

Alkali Canyon

Hartman Draw

Simon Draw

Cash Draw

Lone Pine Lateral

Narraguinnep 

Reservoir

U-Lateral

Alkali Canyon

Hartman Draw

Simon Draw

Cash Draw

Lone Pine Lateral

Narraguinnep 

Reservoir

U-Lateral

Alkali Canyon

Cross Canyon

Hartman Draw

Dolores River

Navajo Wash

Mud Creek

Beaver Creek

Dawson Draw

Sand Canyon

Yellow Jacket Canyon

Simon Draw

Trail Canyon

Yellow Jacket Canyon

Rock Creek

Sandstone Canyon

Cash Draw

Trail Canyon

House Creek

Dolores River

MONTEZUMA

DOLORES

Lone Pine Lateral

Towaoc Canal

Narraguinnep 

Reservoir

U-Lateral

Figure 4. Irrigated lands in the year 2000 and canal configuration after completion of McPhee Reservoir, southwest 

Colorado. 
8 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

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McElmo Creek

McElmo Creek

Mud Creek

109°00'W 108°30'W

37°30'N

37°15'N

0 5 10 MILES

0 7 14 KILOMETERS

Base from U.S. Geological Survey digital data 1:300,000

Universal Transverse Mercator projection Zone 13 

North American Datum of 1983 (NAD 83)

Monument 

uplift

San Juan 

Mountains

UTAH COLORADO

ARIZONA NEW MEXICO

enlarged 

area

WYOMING

J

! !

Urban area

McElmo Creek basin boundary

EXPLANATION

Figure 5. Surficial exposures of geologic units in the McElmo Creek watershed and surrounding area.
Introduction 9

 GEOLOGY EXPLANATION

 

 UNCONSOLIDATED SURFICIAL DEPOSITS

 AND ROCKS OF QUATERNARY AGE

 

MODERN ALLUVIUM—Includes Piney Creek Alluvium and younger deposits

OLDER GRAVELS AND ALLUVIUMS (PRE-BULL LAKE AGE)—Includes Quaternary 

Verdos, Slocum, Rocky Flats, and Pliocene Nussbaum Alluviums in east, and 

Quaternary Florida, Bridgetimber, and Bayfield Gravels in southwest

EOLIAN DEPOSITS—Includes dune sand and silt and Peoria Loess

LANDSLIDE DEPOSITS—Locally includes talus, rock-glacier, and thick colluvial 

deposits

 IGNEOUS ROCKS OF EARLY TERTIARY 

 AND LATE CRETACEOUS AGE

LARAMIDE INTRUSIVE ROCKS (AGE 40-72? M.Y.)—Mainly intermediate to felsic 

compositions; some mafic

 SEDIMENTARY ROCKS OF CRETACEOUS AGE

CLIFF HOUSE SANDSTONE

MENEFEE FORMATION (SANDSTONE, SHALE, AND COAL) AND POINT 

LOOKOUT SANDSTONE

MANCOS SHALE—Intertongues complexly with units of overlying Mesaverde Group

or Formation; lower part consists of a calcareous Niobrara equivalent and Frontier 

Sandstone and Mowry Shale Members; in areas where the Frontier and Mowry

Members, or these and the Dakota Sandstone are distinguished, map unit (Km) 

consists of shale above Frontier Member

Juana Lopez Member—Calcareous sandstone; a thin but persistent unit distinguished 

only locally

DAKOTA SANDSTONE AND BURRO CANYON FORMATION—Sandstone, shale, 

and conglomerate

 SEDIMENTARY ROCKS OF JURASSIC AGE

MORRISON FORMATION—Variegated claystone, mudstone, sandstone, and local beds

of limestone

MORRISON FORMATION, SUMMERVILLE FORMATION (SHALE AND SILTSTONE),

AND ENTRADA SANDSTONE

MORRISON, WANAKAH, AND ENTRADA FORMATIONS

 SEDIMENTARY ROCKS OF JURASSIC AND TRIASSIC AGE

GLEN CANYON GROUP AND CHINLE FORMATION—In southwest, Glen Canyon 

Group consists of Navajo Sandstone, Kayenta Formation (red siltstone, shale, and 

sandstone), and Wingate Sandstone; Chinle is red siltstone

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10 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

(Western Regional Climate Center, 2008). Because of the lack 

of precipitation in the study area, irrigation is important to 

sustain a productive growing season. 

The Dolores Project

The Dolores Project was designed to provide water for 

irrigation, municipal and industrial use, power production, 

recreation, and wildlife enhancement to Montezuma and 

Dolores Counties in southwest Colorado. McPhee Reservoir 

was constructed as the primary storage device of the project 

(Bureau of Reclamation, 1988). The McElmo Creek basin was 

added to the Dolores Project in 1984 by Public Law 98–569, 

allowing for salinity-control features to be constructed in the 

basin to reduce potential salinity increases in McElmo Creek 

resulting from McPhee Reservoir (Bureau of Reclamation, 

1988). Salinity control was important to the McElmo Creek 

basin because storage in McPhee Reservoir increased the 

length of the irrigation season by supplying increased lateseason irrigation water. The longer irrigation season increases 

both the amount of irrigation water applied and the amount of 

time water is in contact with saline soils which, in turn, can 

increase the salinity load entering McElmo Creek.

Groundbreaking for the Dolores Project occurred in September 1977; however, construction of McPhee Dam did not 

begin until 1980. McPhee Dam was completed and McPhee 

Reservoir began to fill in 1984; by 1986 a full supply of irrigation water was available for use. Construction of salinitycontrol features in McElmo Creek basin began shortly after. 

By 1994, the USBR had completed realignment of the Towaoc 

Canal and lateral system to serve the land previously watered 

by the Rocky Ford Ditch and Highline Ditch. Sections of the 

Hermana and Lone Pine Laterals were also lined in 1994. Offfarm remediation by the USBR ended in 1994 along with the 

completion of the Dolores Project modifications and salinitycontrol features (fig. 6).

 Salinity-control features included in the Dolores Project 

were designed to help reduce and offset the increase in salinity 

load from increased water usage and longer irrigation seasons. 

According to revised estimates by the USBR for the 1977 

Dolores Project Definite Plan Report, an additional 43,150 

tons of salt would enter McElmo Creek annually from project 

lands and canals. Realignment of the Towaoc Canal would add 

another 7,500 tons per year, making a total of 50,650 tons per 

year as a result of plan development (table 1). Salinity-control 

features, including the realignment of the Towaoc Canal, and 

project modifications were estimated to decrease the salinity 

load by 32,000 tons per year. Therefore, the net total project 

effect was estimated to increase the salinity load by 18,650 

tons per year. These estimates do not include any on-farm 

irrigation controls for decreasing salinity loading (Bureau of 

Reclamation, 1988). 

Methods of Analysis

Site Selection 

Sites in the McElmo Creek study area were selected by 

their location relative to McElmo Creek and the amount of 

available data associated with each site. Sites that lacked a 

continuous data record or had missing values that could not 

be estimated using regression analysis for either the preperiod or the postperiod were not used in the data analysis. 

The USBR, USGS, and CDWR operated sites in and near the 

McElmo Creek basin that were used in the analysis (table 2; 

fig. 1). Data for the USBR sites were used mostly for analysis 

of preperiod streamflow and water quality, whereas the USGS 

data were used mostly for analysis of postperiod streamflow 

and water quality. The CDWR data were used for determination of canal flow and diversion amounts for both preperiod 

and postperiod. 

From July 1972 to February 1984, streamflow and waterquality data were collected by the USBR before and during the 

implementation of the salinity-control features of the Dolores 

Project. Data for this period, including field measurements and 

laboratory analyses, were extracted by physical examination of 

12 volumes of historical hardcopy records (on file at Bureau of 

Reclamation, Durango office, Durango, Colo.). The extracted 

data were entered into spreadsheet format for analysis and 

preservation.

Quality assurance for site name and location was established by importing the data into a Geographic Information 

Systems (GIS) database using the attribute records of quartersection, township, range, and local-vicinity description. Sites 

with inconsistent or changed names were assigned a single 

unique name and an explanatory remark was entered in the 

database. Any modifications of names and locations made 

for this study were verified by USBR personnel associated 

with the Dolores Project sampling activity. Sites duplicating 

an existing USGS site were assigned a USGS name, a station 

number, and a remark code in the database. Records without 

a verified location and date were removed from the active 

dataset. 

The records in the active dataset were required to meet 

minimum criteria for salinity-load calculations. Each record 

had to contain a daily streamflow value and either a daily TDS 

value or a daily specific-conductance (SC) value. Specific 

conductance is a measure of the ability of water to conduct an 

electrical current and can be used to estimate TDS (Mills and 

others, 1993). 

USGS water-quality sampling sites within the McElmo 

Creek basin and adjacent drainages were identified. The 

water-quality data were retrieved from the USGS National 

Water Information System (NWIS) database for selected sites 

at http://nwis.waterdata.usgs.gov/nwis. Water-quality data 

consisted of daily mean streamflow and daily mean SC values. Water-quality data were analyzed using standard USGS 
1965 1970 1975 1980 1985 1990 1995 

Sept. 30, 1968: Dolores Project authorized. Public Law (90-537)

June 24, 1974: Colorado River BasinSalinity Control Act. Public Law (93-320)

Sept. 24, 1977: Groundbreaking ceremony

1980: Construction of McPhee Dam begins

1984: McPhee Dam completed,begins to fill

Oct. 30, 1984: Public Law (98-569)Salinity Contol on McElmo Creek authorized

1986: Montezuma ValleyIrrigation Company (MVIC)has access to full water supply

March 1992: Old MVIC Highline Ditchabandoned and new Towaoc Canal in use for irrigation season

March 1992: Reach 1 of Towaoc 

Canal from powerplant to U.S. Hwy. 160 complete

April 1994: Reach 2 of Towaoc Canal from Hwy. 160 to Ute Mountain Reservation complete

April 1994: Rocky Ford Lateralsystem is completed (eight burto serve abandoned Rocky FordDitch land)

April LP-LaHerm

April 19and we

1990: Natural Resources Conservation Service (NRCS) on-farm and off-faimprovements begin. Bureau of Reclamation (USBR) off-farm improveme

Figure 6. Timeline of events related to the completion of the Dolores Project from 1965 through 2004.
12 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Table 1. Initial and updated estimates of salinity loading as a result of the Dolores Project, southwest Colorado.

[Estimates from Dolores Project Report (Bureau of Reclamation, 1988); DPR, Dolores Project Report; USBR, Bureau of Reclamation]

DPR elements

Initial salinity loading 

in 1977 DPR plan, in 

tons per year

Updated salinity loading 

for the 1977 DPR plan, 

in tons per year

Change in salinity loading 

resulting from project 

modifications, 

in tons per year

USBR estimated salinity 

loading for the plan of 

development, 

in tons per year

Dolores Project Area—Projected 

lands and canals

10,0801 43,1502 0 43,150

Towaoc Canal—West 

alignment

0 7,500 –7,500 0

Salinity control features 0 0 –24,500 –24,000

Total project effect 10,080 50,650 –32,000 18,650

1

Original DPR plan did not account for canal seepage.

2

Canal seepage accounted for in salinity load calculation.

Table 2. Water-quality sites used in the analysis of salinity load in the McElmo Creek basin, southwest Colorado.

[Sampling agency: USGS, U.S. Geological Survey; USBR, Bureau of Reclamation; CDWR, Colorado Division of Water Resources; --, no data available]

Site 

number 

(fig. 1)

Sampling site 

identifier Sampling site name Latitude Longitude Sampling agency

Period of 

sampling activity, 

in water years

1 09372000 McElmo Creek near Colorado-Utah 

State line

37°19'22'' 109°00'54'' USGS

USBR

1977–2006

1972–1989

2 09371700 McElmo Creek below Cortez, CO 37°19'22'' 108°40'21'' USGS

USBR

1977–2006

1972–1989

3 09371002 Navajo Wash near Towaoc, CO 37°12'03'' 108°41'50'' USGS

USBR

1986–1994

1982–1983

4 09371520 McElmo Creek above Trail Canyon 

near Cortez, CO

37°19'36'' 108°42'00'' USGS

USBR

1990–2006

--

5 09371500 McElmo Creek near Cortez, CO 37°19' 22'' 108°40'21'' USGS

USBR

1982–1993

1978–1982

6 09371492 Mud Creek at State Hwy 32 near 

Cortez, CO

37°18'46'' 108°39'38'' USGS

USBR

1982–2006

1973–1978

7 09371495 Mud Creek near Cortez, CO 37°19'10'' 108°40'03'' USGS

USBR

1978–1981

1972–1983

8 09371420 McElmo Creek above Alkali Canyon 

near Cortez, CO

37°19'38'' 108°38'55'' USGS

USBR

1977–1986

1972–1982

9 09371400 Hartman Draw at Cortez, CO 37°19'26'' 108°36'52'' USGS

USBR

1978–1990

1978–1982, 2007

10 LPHB Lone Pine Canal at Hwy Bridge 37°28'57'' 108°38'20'' CDWR 1974–2006

11 UCANALCO U-Lateral Canal below Great Cut 37°30'45'' 108°35'52'' CDWR 1977–2006

12 -- Lone Pine Lateral 37°30'42'' 108°35'29'' USBR 1978, 1986–2006

13 -- U-Lateral Montezuma Valley Irrigation 

Company

37°30'43'' 108°37'27'' CDWR 1981–2006

14 -- Main Canal #2 37°28'10'' 108°31'47'' USBR 1977–1984

15 DOLTUNCO Dolores Tunnel Outlet near Dolores, CO 37°28'00'' 108°32'01'' CDWR 1985–2006

16 B100 Main Canal #1 37°28'08'' 108°31'43'' USBR 1977–1985

17 09166500 Dolores River at Dolores, CO 37°38'21'' 108°29'49'' USGS

USBR

1977–2006

18 -- Summit Reservoir Outlet at Hwy 37°25'32'' 108°23'43'' USBR 1979–1981
Methods of Analysis 13

techniques and procedures (U.S. Geological Survey, variously dated). Sites relevant to the study area were categorized 

by date of available data, location, and quality of data. Data 

analysis included 10 USGS sites containing 1,728 discrete 

water-quality samples intermittently collected from water year 

1978 to 2006.

Data collected by the CDWR were obtained through the 

CDWR online archived-data retrieval program (http://www.

dwr.state.co.us/SurfaceWater/data/rtarchive.aspx, accessed 

January 10, 2008). Daily mean streamflow records of locally 

operated canals were obtained through the Colorado Decision Support System (CDSS) online Flood DSS Map Viewer 

(http://165.127.23.41/website/FloodIMS/viewer.htm, accessed 

January 10, 2008) and CDSS Stream Flow Data Selector 

(http://cdss.state.co.us/Streamflow/StreamFlow.aspx, accessed 

January 10, 2008). Questions regarding station names, locations, and streamflow discrepancies due to transbasin water 

diversions were directed to the CDWR, Water Division 7, San 

Juan/Dolores River basins branch-office water commissioner 

in Durango, Colo. Data analysis included continuous daily 

mean values for water years 1978 to 2006 for four CDWR 

sites. A record of collected samples used in this report can 

be obtained from the USGS Colorado Water-Quality Data 

Repository online at http://rmgsc.cr.usgs.gov/cwqdr/

Southwest/index.shtml. 

Data Analysis

Multivariate Regression Models

Multivariate regression models were developed for 

sites along McElmo Creek to calculate daily streamflow, SC, 

and TDS concentration, loads, and trends within the dataset. 

Regression analysis is useful to determine relations between 

multiple variables. Estimations of values for a response (y) 

variable can be calculated based on the values of a second 

explanatory (x) variable (Helsel and Hirsch, 2002). Response 

variables are the output value of the equation and are dependent of the explanatory variable, which is the input variable 

and acts independent of the response variable. Explanatory 

variables considered for the regressions included streamflow, 

SC, and a seasonal or periodic variable. A continuous dataset 

allowed for the use of daily or monthly regression outputs to 

define load. Daily values help to more accurately predict load. 

The regressions were developed using periodic data collected 

in the field. Continuous SC data were not always available for 

estimating TDS values. Therefore, gaps in the SC record were 

predicted from SC data available at an adjacent site by using a 

maintenance of variance extension, type 1 (Hirsch, 1982). 

In order for a linear regression to be developed, assumptions had to be made about the representative sample population of the dataset for a given site. To accurately predict a 

response variable, the relation between response variable and 

explanatory variable needs to be linear and residuals normally 

distributed with similar variance over a range of predicted 

values. Typical plots of streamflow relative to TDS have a 

curvilinear shape as a result of a variable TDS-concentrationto-streamflow ratio at different times of the year. To normalize 

a regression model, the data of x, or y, or both x and y need to 

be transformed to eliminate the curvature and heteroscedacity 

in the data (Helsel and Hirsch, 2002). Transformation of the 

McElmo Creek streamflow data was required to remove curvature and heteroscedacity. A natural-log transformation was 

chosen to create an acceptable linearization of the data. The 

data used to fit the model also need to represent the data of 

interest (Helsel and Hirsch, 2002). Residual plots (where the 

residual is the estimated value minus the correlating observed 

value) are necessary to determine whether a normal distribution is present. A normal distribution is important when testing 

the hypothesis for confidence.

The multivariate regression models also included a 

seasonal term. The seasonal term was applied to adjust (or 

account) for the relation between streamflow and the irrigation season of McElmo Creek region. The seasonal variable 

is based on a Julian date and herein is referred to as a dummy 

variable. The dummy variable is assigned a value of 0 for the 

irrigation season and a value of 1 for the nonirrigation season. 

The irrigation season in the McElmo Creek region typically 

is from April through October, and the nonirrigation season is 

from November through March. 

Multivariate regression models were derived using 

ordinary least-squares (OLS) regression with S-PLUS 7.0 

software (Insightful Corp., 2005a). The statistical applicability 

of each model was determined using regression diagnostics 

such as the coefficient of determination (R2), standard error 

of estimate, and p-values, which indicate the probability the 

slope is zero, for individual variables. Models with R2 values 

greater than 0.70 and a homoscedastic distribution of residuals 

were considered acceptable. Models with R2 values less than 

0.70 were rejected and other variable combinations were then 

tested. The larger the p-value, the less likely the variables are 

related. Models with p-values greater than 0.05 were rejected 

because such values indicated an unacceptable correlation 

between variables. Standard error of estimate is an estimate 

of the standard deviation of the residuals about the regression. Smaller values of the standard error of estimate mean 

the representation of the prediction is more accurate (Driver 

and Tasker, 1990). Plots of residuals also were used to help 

identify normality. Residual plots for appropriate models 

show little to no curvature or change in shape. Residual plots 

relative to time also were created to determine if a correlation 

exists. The presence of a correlation indicates that a sampling 

bias or a trend exists (Helsel and Hirsch, 2002). All of these 

diagnostics were used in evaluating regression models and 

determining trends in the dataset.

Salinity-Load Calculations

Salinity load is the amount of salt that is transported by 

a water body; salinity load typically is reported here in units 

of tons per unit of time. Salinity loads were determined as 
14 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

the product of streamflow and the concentration of TDS. Two 

types of TDS data were used in this report, sum of constituents 

(SOC) and residue on evaporation (ROE). SOC is the sum of 

the total soluble anion and cation concentrations. ROE is the 

residue remaining after a given volume of water is evaporated 

for a given amount of time at a given temperature. ROE has 

potential for error due to organic matter that might not be 

completely volatilized, or from the presence of hydrophilic 

residues such as gypsum-laden sulfates (Hem, 1985). When 

applicable, SOC was used preferentially over ROE for regression analysis. 

When daily TDS data were available, daily salinity load 

was calculated using the following equation:

L TDS Q s = ( ) . 0 0027 (1)

where

LS is the daily salinity load, in tons,

TDS is the measured daily TDS concentration, in 

 milligrams per liter,

Q is the daily mean streamflow, in cubic feet 

 per second, and 

 0 0027 . is the unit constant to obtain tons per day from 

 milligrams per liter and cubic feet per second.

When TDS data were not available to use in equation 1, 

a TDS value was estimated using a regression model based on 

SC. 

A TDS value was calculated as a function of SC by using 

the following equation:

TDS SC e = +   0 1 ( ) (2)

where

TDSe is the estimated TDS concentration in 

 milligrams per liter,

SC is the daily mean SC in microsiemens per 

 centimeter at 25° Celsius, and 

n are regression coefficients.

The estimated TDS concentration ( TDSe ) was then substituted 

into equation 1 for daily salinity-load calculations. The form 

of equation 2 used at selected sites in the McElmo Creek basin 

is summarized in table 3.

Missing SC values were estimated by using an upstream 

site with available SC data. The record extension technique, 

Maintenance of Variance Extension, Type 1 (MOVE.1) was 

used. MOVE.1 is an alternative to the standard regression 

approach and maintains the sample mean and variance in 

the dataset. SC data for McElmo Creek above Trail Canyon 

near Cortez (site 4), McElmo Creek near Cortez (site 5), and 

Hartman Draw at Cortez (site 9) were used to determine missing SC values for McElmo Creek near the Colorado–Utah 

State line (site 1). These were chosen to predict SC at site 1 

because they are the closest upstream sites on the main stem 

of McElmo Creek. The site used to predict the SC at site 1 

was determined by SC data availability during the period 

of missing SC values from site 1. When multiple sites had 

overlapping SC data for a needed time period, the closest 

upstream site was used. SC values for missing dates were 

estimated by using the following equation:

SC y Sy

Sx x x e = + −i ( ) (3)

where

SCe is the estimated SC value at site 1, in 

 microsiemens per centimeter at 25° Celsius,

y is the sample mean of observed SC values at 

 site 1,

x is the sample mean of observed values at a

 selected adjacent site,

Sy is the standard deviation of observed values 

 at site 1,

Sx is the standard deviation of observed values 

 at a selected adjacent site, and xi is the observed SC value of a selected adjacent site.

The estimated SC value was then used in equation 2 to estimate the TDS concentration. 

A 3-year moving-average regression equation was also 

used to estimate missing daily SC values. When SC data 

from an adjacent site were not available for use in equation 3, 

equation 4 was used to fill in missing daily values. The 3-year 

moving-average regression uses data for a 3-year period to 

determine the SC value for a given day in the middle year in 

the selected period. The period then moves 1 year forward and 

the calculation is repeated for the next year to be calculated 

(Kircher and others, 1984). Estimations of SC data using a 

3-year moving-average regression were calculated using the 

equation:

1 1 0 1 2 nSCe = + +    nQ D (4)

where

1nSCe is the natural logarithm of specific 

 conductance, in microsiemens per centimeter,

n are regression coefficients,

1nQ is the natural logarithm of daily mean 

 streamflow at McElmo Creek near the 

 Colorado–Utah State line, in cubic feet 

 per second, and

D is a dummy variable where 1 was used for the 

 nonirrigation season and 0 for the irrigation

 season.

The resulting SC value was then used in equation 2 to estimate 

the TDS concentration. By using a predicted value of SC to 

predict TDS, errors are introduced. Karlinger and Troutman 

(1985) indicate that biases in the regression line and error 

variance are a function of lacking a correlation between independent and dependent variables, and errors associated with a 

cascade-regression prediction can be large. 

Estimates of daily salinity loads from Mud Creek, which 

drains into McElmo Creek, were calculated using data from 

two sites, Mud Creek near Cortez (site 7) and Mud Creek at 

State Hwy 32 (site 6). These data were used in equation 1 to 
Methods of Analysis 15

estimate salinity load. Data records for site 6 exist for water 

years 1973–1978 and water years 1982–2006. Data records for 

site 7 are available for water years 1972–1983. Because neither site had a complete preperiod record, data for site 7 were 

used for water years 1978–1981 and data for site 6 were used 

for water years 1982–1984. For the postperiod, data for site 6 

were used exclusively because no data had been collected at 

site 7 since 1983. Data from the two sites were compiled and 

used to calculate the daily salinity loads for Mud Creek. These 

salinity-load estimates are assumed to represent the entire Mud 

Creek basin.

Data for site 7 for water years 1979–1983 were used in 

equation 2 to calculate missing TDS data for the salinity-load 

estimates (table 3). If SC data were missing, equation 3 was 

used. The resulting TDS values were then entered into equation 1 to estimate salinity load. Few streamflow and SC data 

were available for water years 1982–1983; therefore, data 

from site 7 were only used for water years 1979–1981. 

Data from site 6 were used to calculate the daily salinity 

load for water years 1982–2006 (table 3). Daily salinity load 

was determined using equation 1. In the absence of TDS data, 

equation 2 was used to estimate TDS. When SC data were not 

available, the data were calculated using equation 3 and then 

entered into equation 2. The resulting TDS values were then 

entered into equation 1 to estimate salinity load.

The Navajo Wash basin was included in the McElmo 

Creek salinity-load estimations because it collects irrigation 

return water from irrigated lands within the Dolores Project 

area and receives the water that is left in Towaoc Canal at its 

terminus. The decommissioned Rocky Ford Ditch, along with 

the Highline Ditch and the Towaoc Canal, supplied water to 

irrigated lands that had return flow into Navajo Wash. Data 

for USGS streamgaging station 09371002 Navajo Wash near 

Towaoc, CO (site 3) allow for estimation of salinity loads 

discharging from the McElmo Creek basin through Navajo 

Wash. Data associated with site 3 are sparse and mostly 

incomplete. Therefore, salinity-load data during the preperiod 

from water year 1982 to 1984 were estimated from available 

salinity data available from the USBR (on file at Bureau of 

Reclamation, Durango, Colo.). Data for the postperiod were 

sporadic and incomplete, making it very difficult to calculate a 

salinity load. Because of the geographic and geologic similarities of Navajo Wash basin to the Mud Creek basin, the salinity 

load at site 3 was assumed to be similar to that at site 6 for 

both periods. Therefore, the salinity load discharging at site 3 

during the preperiod was multiplied by the postperiod percentchange factor for site 6 as a means to estimate the salinity load 

discharging at site 3 for the postperiod. The salinity load for 

site 3 for the preperiod was calculated using equations 1–4 as 

shown herein. 

Trend Analysis

The S-Plus load-computing program Load Estimator 

(S-LOADEST) was used to assist in the evaluation of time 

trends in the dataset. S-LOADEST calculates loads based on 

a model developed by the user. The load outputs and residuals can then be evaluated for trends. Plots of the residuals are 

checked for homoscedacity, linearity, and constant variance to 

identify a model of best fit. 

Plots of the partial residuals can be used to assess trends 

within a dataset. Partial residuals are the difference between an 

observed value of y and a predicted value of y from a regression equation where an explanatory variable of interest (such 

as time) is left out of the model, while all other explanatory 

variables are present. The partial residual plot describes the 

relation between y and the explanatory variable left out of the 

model after all effects of the other explanatory variables have 

been removed (Helsel and Hirsch, 2002). 

A LOcally WEighted Scatterplot Smoothing (LOWESS) 

curve can also be used to evaluate whether a trend exists in the 

partial residual data set. The LOWESS curve can be useful in 

indicating where changes in the trend occur that are not visible 

just from analyzing data points in the partial residual plot. 

LOWESS is an approach to smoothing using a noise-reduction 

algorithm (Insightful Corp., 2005b). LOWESS describes the 

Table 3. Summary of equations used to estimate TDS at selected sites in the McElmo Creek region, southwest 

Colorado.

[TDS, total dissolved solids; ln, natural logarithm; SC, specific conductance; R2, coefficient of determination]

Site number

(fig. 1)

Period of 

record used Equation Intercept Coefficient R2

1 1978–1983 TDS = 0.8722(SC) – 61.363 –61.363 0.8722 0.832

1984–1999 TDS = 0.9771(SC) – 354.71 –354.71 0.9771 0.950

2000–2006 TDS = 0.9687(SC) – 371.81 –371.81 0.9687 0.993

3 1987–1994 TDS = 0.9897(SC) – 465.19 –465.19 0.9897 0.991

6 1982–1986 lnTDS = 1.1436(ln SC) – 1.3519 –1.3519 1.1436 0.991

1994–2006 lnTDS = 1.1506 (ln SC) – 1.4058 –1.4058 1.1506 0.997

7 1979–1981 TDS = 1.2127(SC) – 869.319 –869.319 1.2127 0.987

15 1978–2006 TDS = 0.5999(SC) – 0.3666 –0.3666 0.5999 0.932
16 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

relation between x and y variables, and the partial residuals 

are not assumed to be linear or normalized. LOWESS smoothing is a robust description of the pattern formed by the data 

(Helsel and Hirsch, 2002). When homoscedacity, linearity, 

and constant variance are met, linear trends in the dataset are 

assessed. S-LOADEST can then be used to calculate the load 

for a beginning year of choice and an ending year of choice; 

the difference in load of the selected years can be used to estimate any upward or downward trend that might exist for the 

calculated load based on time (TIBCO, 2008). 

Data for sites 1, 6, and 17 were analyzed for trends 

during the study period from water years 1978 to 2006. 

Regression models were calibrated in the statistical software 

S-LOADEST based on the FORTRAN version developed by 

Runkel and others (2004). The equation for each site was calibrated using data for all available discrete, intermittent waterquality samples within the study-period timeframe. Initially a 

seven-parameter model was used in this report, incorporating 

flow dependence, time trends, and seasonality. Accounting for 

differences in seasonality can also be accomplished using an 

irrigation season “dummy variable” and a pair of trigonometric variables that define an annual cycle. The following sevenparameter model was used:

1 1 1 1 1 0 1 2

2

3

4

2

5

nL nQ Q Q Q t t n n n

t t

 = + − + − + −

+ − +

   

 

( ) ( ) ( )

( ) si

* * *

* n( ) cos( ) K T K T D 2 2     + 6 + 7

 (5)

where

1nL

 is the natural logarithm of predicted salinity load,

 in tons per day;

0 is the regression equation intercept;

n is the coefficient for the regression variable n;

1nQ is the natural logarithm of daily mean streamflow,

 in cubic feet per second;

1nQ* is the natural logarithm of streamflow, 

 centered for the calibration dataset, 

 in cubic feet per second;

t is time, in decimal years;

t

* is the time, centered for the calibration 

 dataset, in decimal years; 

sin( ) K T 2 and cos( ) K T 2 are seasonality terms, where K

is an integer and T is the decimal portion 

 of the year starting January 1; and

D is a seasonality “dummy” variable for irrigation

 and nonirrigation seasons.

Additional information can be found in Cohn (2005).

Explanatory variables were orthogonalized to “center” 

the data and remove any multicolinearity of the explanatory 

variables, which can create load estimation errors (Runkel and 

others, 2004). These data were “centered” using the following 

equations described by Cohn and others (1992):

1 1

1 1

2 1 1

3

1

2

1

n n

n n

n n

Q Q

Q Q

Q Q

i

i

N

i

i

N

*

( )

( )

= +

−

−

=

=

∑

∑ and 1

1

1 n

n

Q

Q

N

i

i

N

= =

∑

 (6)

where

1nQ* is the natural logarithm of streamflow, centered

 for the calibration dataset, in cubic 

 feet per second;

1nQ is the mean of the natural logarithm of 

 streamflow in the dataset, in cubic feet 

 per second;

1nQi is the natural logarithm of daily mean 

 streamflow for day i, in cubic feet per 

 second; and

N is the number of daily values in the dataset.

t t

t t

t t

i

i

N

i

i

N

*

( )

( )

= +

−

−

=

−

∑

∑

1

3

2

1

2

 and t

t

N

i

i

N

= =

∑1 (7)

where

t

* is the time, centered for the calibration 

 dataset, in decimal years;

t is the mean of the time in the dataset, in 

 decimal years;

ti is time for day i, in decimal years; and

N is the number of daily values in the dataset.

If logarithmic transformations are used for the response 

variable, a bias is produced when the logarithms of the estimated values are retransformed to original units. An underestimation of the mean response typically results, and the majority 

of the bias can be eliminated by multiplying the estimated 

response by a bias-correction factor (Duan, 1983). This bias 

correction was done by the S-LOADEST software, and no 

manual calculation was necessary. For details on transformation and bias-correction factors see Runkel and others (2004), 

Cohn and others (1989), and Likes (1980).

The S-LOADEST program estimates regression coefficients using an Adjusted Maximum Likelihood Estimation 

(AMLE) method (Cohn, 1989). This method gives the same 

results as standard MLE method if there are no censored 

values in the dataset. S-LOADEST calculates the loads based 

on the calibrated regression model for each site. S-LOADEST 

also calculates a salinity concentration from the estimated load 

by dividing the load by the flow value and conversion factor. Salinity loads and concentrations were estimated for the 

entire study period (water years 1978–2006) and for a period 

of time designated as having high levels of salinity-control 

activity and expected visible trends (water years 1989–2006). 

A summary of the best-fit model, which was derived from the 

seven-parameter model, used for sites 1, 6, and 17 is given in 

table 4. Statistical diagnostics are summarized in table 5. An 

important note is that centered SC was a significant explanatory variable in the regression equations but was not used in 

the S-LOADEST equations because mean daily values for 

SC were not available for site 1 and site 17, and SC was not a 

significant variable for site 6. 
Characterization of Hydrology and Salinity in McElmo Creek 17

Table 4. Summary of equation forms used to estimate salinity load at the sampling sites.

[L, salinity load, in tons per day; B0, coefficient that is the y-axis intercept in the regression model; Bn, estimated coefficients of the explanatory variable; Q, 

daily mean streamflow, in cubic feet per second; Q*, streamflow centered for the calibration dataset, in cubic feet per second; t, time, in decimal years; t*, 

time centered for the calibration dataset, in decimal years; T, seasonality term representing the decimal portion of the year; K, an integer]

Site 

number 

(fig. 1)

Response variable Explanatory variable

Equation form Variable Transformation Variable Transformation

1 L Natural log Q,Q*

t, t*,T

Natural log

None

lnL = B0+B1(lnQ-lnQ*)+B2(t-t*)+B3sin(2πT)+B4cos(2πT)

6 L Natural log Q,Q*

t, t*,T

None lnL = B0+B1(lnQ-lnQ*)+B2(t-t*)+B3sin(2πT)+B4cos(2πT)+B5sin

 (K2πT)+B6(K2πT)

17 L Natural log Q,Q*

t, t*,T

Natural log

None

lnL = B0+B1(lnQ-lnQ*)+B2(t-t*)+B3sin(2πT)+B4cos(2πT)+B5sin

 (K2πT)+B6(K2πT)

ˆ

ˆ

ˆ

ˆ

Table 5. Summary of regression model coefficients and diagnostics for three sampling sites.

[Q, daily mean streamflow, in cubic feet per second; Q*, streamflow centered for the calibration dataset, in cubic feet per second; T, seasonality term representing the decimal portion of the year; K, an integer; t, time, in decimal years; t*, time centered for the calibration dataset, in decimal years; ERV, estimated 

residual variance; SCR, serial correlation of the residuals; R2, coefficient of determination; --, no data]

Seasonality variables

Fourier series

Streamflow 2πT K2πT Decimal 

time

Statistical 

diagnostics

Site 

number

Y-axis 

intercept lnQ lnQ* Sin Cos Sin Cos Dummy variable t t* ERV SCR R2

1 5.0303 0.8340 26.06 0.1240 0.2655 -- -- -- –0.0141 1991.86 0.0156 0.233 97.55

6 3.6446 0.7332 4.598 0.0781 0.3098 –0.1304 –0.0277 -- –0.0457 1991.21 0.0264 0.1329 92.79

17 5.0259 0.7546 340.5 0.0455 0.1289 0.0186 –0.04023 -- –0.0047 1990.57 0.0179 0.0609 98.51

Outliers also were evaluated and compared to the rest of 

the dataset. Outliers that were not consistent with the normal 

flow regime, such as high streamflows resulting from large 

rainstorms that created abnormal data points, were removed 

from the dataset in order to prevent a false trend. One outlier 

was removed from the site 6 dataset and one removed from the 

site 17 dataset because they appeared to be the result of monsoonal-type rainstorms that did not fit the rest of the dataset.

Characterization of Hydrology and 

Salinity in McElmo Creek

The streamflow regime of McElmo Creek is based largely 

on seasonal moisture patterns, along with regulated releases 

from Summit Reservoir and McPhee Reservoir. Typical with 

other alpine headwater streams, seasonal streamflow is dependent on snow moisture content and the timing and intensity 

of rainfall and snowmelt. Water quality in McElmo Creek 

is affected by seasonal changes in streamflow pattern. TDS 

concentration changes frequently with changing flow conditions, moisture patterns, and regulated releases from McPhee 

Reservoir and Summit Reservoir. Dilution occurs when 

increased water is supplied to the system. Dilution typically 

occurs during spring runoff and intense storms. However, 

increasing the amount of water to the system also can flush 

salts into the stream and can create large increases in TDS 

from the dissolution of near-surface salt deposits.

Hydrology

Streamflow patterns vary throughout the McElmo Creek 

region. Streamflow data are summarized in table 6. Streamflow rate in McElmo Creek fluctuates as water moves downstream. Irrigation withdrawals decrease flow in the stream, 

whereas water gained from tributaries increases flow. Mean 

daily streamflow for the McElmo Creek region for the study 

period ranged from 6.31 ft3/s at site 7 to 57.3 ft3/s at site 5 

(fig. 7, table 6). Daily streamflow values for study sites in the 
18 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Table 6. Summary of daily streamflow data for selected steamgaging stations in or near the McElmo Creek basin, southwest 

Colorado.

[Site name, U.S. Geological Survey (USGS) station name; ft3/s, cubic feet per second; Max, maximum; Min, minimum; # Obs, number of 

observations]

Daily streamflow, ft3/s

Site name

Site 

number 

(fig.1)

USGS site 

identifier Period of record Max Min Mean Median # Obs

Dolores River at 

Dolores, CO

17 09166500 1900/01/01–1903/10/01, 

1910/10/01–1912/19/30, 

1921/10/01–2005/12/28

6,950 11.00 434 123 32,900

Hartman Draw at 

Cortez, CO

9 09371400 1978/04/01–1986/09/30 104 0.28 13.6 11.0 3,110

McElmo Creek 

above Alkali 

Canyon, near 

Cortez, CO

8 09371420 1972/10/01–1986/09/30 302 1.50 27.4 20.0 5,110

McElmo Creek 

above Trail 

Canyon near 

Cortez, CO

4 09371520 1993/08/01–2005/12/28 1,200 2.73 55.9 43.0 4,530

McElmo Creek 

below Cortez, CO

3 09371700 1972/10/01–1983/09/30 470 0.04 41.6 28.0 4,020

McElmo Creek near 

Colorado–Utah 

State line

1 09372000 1951/03/01–2005/12/28 1,220 0.08 50.1 37.9 20,000

McElmo Creek near 

Cortez, CO

5 09371500 1926/10/01–1929/09/30, 

1940/10/01–1943/09/30, 

1950/10/01–1954/09/30, 

1982/01/01–1993/09/30

1,480 2.00 57.3 47.0 7,940

Mud Creek at State 

Highway 32, near 

Cortez, CO

6 09371492 1981/10/01–1986/09/30, 

1993/08/01–2005/12/28

230 0.23 7.43 4.30 6,360

Mud Creek near 

Cortez, CO

7 09371495 1978/04/01–1981/09/30, 

1983/10/01–1984/09/30

154 0.03 6.31 3.60 1,650

Navajo wash near 

Towaoc, CO

3 09371002 1986/10/01–1988/09/30, 

1989/04/01–1994/09/30

100 0.33 8.59 6.15 2,740

region ranged from 0.03 ft3/s at site 7 to 6,950 ft3/s at site 17 

(table 6). 

Site 1, McElmo Creek near Colorado–Utah State line, is 

the farthest downstream site used to determine water quality for this report and is generally a good indicator of the 

quality of water discharging from the McElmo Creek basin 

(fig. 1). The maximum and minimum daily streamflows were 

1,220 ft3/s and 0.08 ft3/s, respectively, over a period of record 

from March 1, 1951, through December 28, 2005. The mean 

and median daily streamflows for this period were 50.1 ft3/s 

and 37.9 ft3/s, respectively (table 6). Figure 7 shows the daily 

streamflow for site 1 and the water year annual total in acrefeet. The graphs show low daily and annual flows during the 

drought years 1977, 2002, and 2003. 

The dominant pattern of daily streamflow in McElmo 

Creek consists of high flows in the spring as a result of 

snowmelt and runoff, followed by a decrease in streamflow 

throughout the irrigation season and then an increase in 

streamflow in the fall and winter as irrigation ceases and precipitation is more frequent. Streamflow patterns for tributaries 

to McElmo Creek are influenced substantially by irrigation 

and typically would be ephemeral if irrigation was not present 

in the region. However, because irrigation is present, streamflows typically increase during the snowmelt period and 

remain relatively high during the irrigation season, and then 

recede as irrigation ceases for the season. 
Site 1

0

200400600800

1,0001,200

1951-03

1952-04

1953-05

1954-06

1955-07

1956-08

1957-09

1958-10

1959-12

1961-01

1962-02

1963-03

1964-04

1965-05

1966-06

1967-08

1968-09

1969-10

1970-11

1971-12

1973-01

1974-02

1975-04

1976-05

1977-06

1978-07

1979-08

1980-09

1981-10

1982-12

1984-01

1985-02

1986-03

1987-04

1988-05

1989-06

1990-08

1991-09

1992-10

1993-11

1994-12

1996-01

1997-02

Date

5,000

15,00025,00035,00045,00055,00065,000

1952

1953

1954

1955

1956

1957

1958

1959

1960

1961

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

Water Year

Streamflow, in acre-feet per year Streamflow, in cubic feet per second

A

B

Figure 7. (A) Daily streamflow for site 1, McElmo Creek near Colorado–Utah State line, from March 1951 to December 2005. (B) Streamflow in acre

McElmo Creek near Colorado–Utah State line, for water years 1952–2005.
20 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Salinity

Salinity concentrations and loads for the McElmo Creek 

basin were estimated and summarized for watershed inputs 

and outputs. The daily mean salinity concentrations were 

calculated for each water year of the preperiod and postperiod 

to analyze the effects of salinity-control efforts associated with 

the Dolores Project. Salinity-concentration data contained 

annual daily mean summary statistics for the 7-year preperiod 

(water years 1978–1984) and the 7-year postperiod (water 

years 2000–2006). Preperiod and postperiod median values 

were compared between corresponding input and output sites 

in the basin.

Daily mean salinity concentrations are summarized in 

table 7 for the preperiod and the postperiod. The values in 

table 7 are represented in graphical form in figure 8 as box 

plots comparing the preperiod and postperiod for salinity 

concentration. The main inflow source of irrigation water 

to McPhee Reservoir and the McElmo Creek basin is monitored at site 17, Dolores River at Dolores, CO. Daily salinity 

concentrations for water in McPhee Reservoir outlet canals are 

very similar or identical to those for the Dolores River; therefore, concentration values from the Dolores River at Dolores 

were used to fill in any missing daily salinity values for the 

basin input canals. 

Preperiod and postperiod salinity concentrations at 

selected sites in the McElmo Creek basin are compared in 

figure 8. Salinity concentrations decreased from the preperiod 

to the postperiod in the main basin outflow. The median salinity concentration for site 1 (main basin outflow) decreased 

from 2,210 mg/L per day in the preperiod to 2,110 mg/L per 

day in the postperiod. However, the median salinity concentration for site 6 (tributary outflow) increased from 3,370 

mg/L per day in the preperiod to 3,710 mg/L per day in the 

postperiod. Salinity concentrations typically increased from 

the preperiod to the postperiod at inflow sites. Median salinity 

concentration increased from 178 mg/L per day during the 

preperiod at site 16 to 227 mg/L per day during the postperiod 

at site 15. 

The reader is cautioned that the data represented above 

are not flow adjusted (normalized). Decreases or increases of 

streamflow could be associated with droughts or major precipitation events and could have an effect on salinity concentration. Non-flow-adjusted data were selected to be represented 

here to add insight to trends in salinity concentration during 

the study period. 

Effect of Irrigation-Delivery and Salinity-Control 

Work by the Dolores Project

Salinity trend analysis (non-flow-adjusted) for the preperiod and postperiod indicates a change in salinity load after 

completion of the reservoir. Data analysis indicates the salinity 

load at site 1 was approximately 772,000 tons for the 7-year 

preperiod (water years 1978–1984) (table 8). Navajo Wash 

(site 3) carried approximately 52,700 additional tons of salt 

out of the basin. The total salinity load exiting the McElmo 

Creek basin was about 824,700 tons during the preperiod. 

Inputs of salinity load to the watershed were calculated using 

data for sites 10, 13, and 14 combined; site 16; and site 18. 

Salinity load entering the basin at these sites was estimated 

to be about 82,700 tons, 77,100 tons, and 6,990 tons, respectively, during the preperiod. Therefore, the total salinity load 

entering the basin at inflow sites was about 167,000 tons during the preperiod. The difference between the inflow salinity 

load and the outflow salinity load indicates the number of tons 

of salt picked up within the basin. Therefore, the amount of 

salinity added in McElmo Creek basin was 657,700 tons during the preperiod (table 8).

 Analysis of postperiod data indicates the salinity load 

leaving the McElmo Creek basin at site 1 was 475,000 tons 

over the 7-year period from 2000 through 2006. An additional 

38,700 tons left the basin through Navajo Wash (site 3). The 

total salinity load exiting the basin during the postperiod was 

approximately 513,700 tons. Inflow sites for the postperiod 

were site 11, site 12, site 15, and site 18. The salinity load 

entering the basin at each of these sites were 7,900, 91,800, 

136,000, and 4,560 tons, respectively, from 2000 to 2006. 

Total inflows of salinity load for the postperiod were 240,000 

tons. Therefore, the total amount of salinity added within 

the basin during the postperiod was 273,700 tons (table 8), 

which is 384,000 tons less than during the preperiod. Figure 9 

illustrates the preperiod and postperiod salinity loads together 

and allows for comparison of preperiod and postperiod loads 

at each site. 

A summary of the water and salinity balance from the 

preperiod and the postperiod is shown in table 8. The percentage of change that occurred at each inflow site and outflow site 

from the 1978–1984 preperiod to the 2000–2006 postperiod 

also is shown. The amount of inflow increased from 909,000 

acre-ft during the preperiod to 983,000 acre-ft during the postperiod. Outflows decreased from 298,000 acre-ft during the 

preperiod to 214,000 acre-ft during the postperiod. Typically, 

the inflows of salinity load were higher in the postperiod than 

they were in the preperiod. The outflows typically decreased in 

salinity load from the preperiod to the postperiod. Decreases in 

salinity load were as much as 38 percent at site 1 and 27 percent at site 3. Total salinity load, in tons per period, for inflow 

sites increased from 167,000 tons for the preperiod to 240,000 

tons for the postperiod, which is a 44-percent increase in salinity load from inflow sites. The total salinity load, in tons per 

period, for outflow sites decreased from 824,700 tons during 

the preperiod to 513,700 tons during the postperiod, which is 

a 38-percent decrease in salinity load from the outflow sites 

(table 8). 

The reader is cautioned that the decreases and increases 

in salinity load are not flow adjusted and are a function of the 

raw data. Therefore, seasonal and climatological effects are 

inherent in the analysis. Adjustments for wet years in the mid1980s and drought years in 2002 and 2003 were not included 
Table 7. Summary statistics for daily mean salinity concentration for selected sites in the McElmo Creek region from water years 1978–19water years 2000–2006 (postperiod).[Fig., figure; No. of obs, number of observations; Min, minimum value; Max, maximum value; Concentration, total dissolved solids concentration, not flow adjusted; mg/Ldata]

Daily salinity concentration, in mg/L

October 1977 through September 1984 October 1999 through Septem

Site no. 

(fig. 1) Site name

No. 

of 

obs

Min 25th percentile Mean/Median 75th percen- tile Max No. of obs Min 25th percen- tile Mean/Med

1 McElmo Creek

near ColoradoUtah state line

2,557 560 1,610 2,200/2,210 2,711 4,390 2,557 655 1,360 2,030/2

6 Mud Creek at 

State Hwy 32 near Cortez, CO

2,192 212 2,160 3,450/3,370 4,590 7,620 2,557 280 1,330 3,370/3

15 Dolores Tunnel 

near Dolores, CO

-- -- -- --/-- -- -- 2,557 75.5 177 225

16 Main Canal #1 2,042 72.8 127 190/178 255 322 -- -- -- --/-- 17 Dolores River 

at Dolores, CO

2,557 73.0 145 204/208 270 322 2557 75.5 177 225
22 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

2557

EXPLANATION

Outlier

Number of observations

Daily mean salinity concentration, water years 1978–1984 (preperiod)

Daily mean salinity concentration, water years 2000–2006 (postperiod)

Upper adjacent

75th percentile

Median

25th percentile

Lower adjacent

0

2,000

4,000

6,000

8,000

10,000

12,000

2192 2557

Site 1 Site 6

0

1,000

2,000

3,000

4,000

5,000

2557 2557

x

x

0 0

100

200

300

400

500

2557 2557

Site 17

100

200

300

400

500

2042 2557

Site 16 (preperiod), Site 15 (postperiod)

Salinity concentration, in milligrams per liter

Figure 8. Boxplots showing distribution of daily mean salinity concentrations at various sites in McElmo Creek basin, 

southwest Colorado.
Characterization of Hydrology and Salinity in McElmo Creek 23

Table 8. Salinity balance with comparisons of preperiod and postperiod salinity loads for inflow and outflow sites.

[Study period: water years 1978–1984 preperiod; water years 2000–2006 postperiod; acre-feet, total acre-feet of discharge for 7-year period; tons per period, 

total tons of salt per 7-year period]

Preperiod (1978-1984) Postperiod (2000-2006) Percent change

Site 

number 

(fig. 1)

Discharge, 

in acre-feet/

period

Salinity load, 

in tons/period

Salinity load, in 

tons/acre-foot

Discharge, in 

acre-feet/period

Salinity load, 

in tons/period

Salinity 

load, in tons/

acre-foot

Tons per 

period 

(%)

Tons per 

acre-foot 

(%)

Outflow sites

1 277,000 772,000 2.78 198,000 475,000 2.40 -38 -14

3 21,100 52,700 2.50 16,000 38,700 2.42 -27 -3.2

Total 298,000 824,700 2.77 214,000 513,700 2.40 -38 -13

Inflow sites

10,13,141 419,000 82,700 0.197 -- -- -- 212 302,3

11 -- -- -- 31,700 7,900 0.249

12 -- -- -- 351,000 91,800 0.262

15 -- -- -- 564,000 136,000 0.241 764 364

16 435,000 77,100 0.177 -- -- --

18 55,200 6,990 0.127 36,000 4,560 0.127 -35 0.0

Total 909,000 167,000 0.184 983,000 240,000 0.244 44 32

1

Combined data for three sites, Lone Pine Canal at Highway Bridge (site 10), U-Lateral Montezuma Valley Irrigation Company (site 13), and Main Canal 

#2 (site 14) into one input site.

2

Sites 10, 13, and 14 represented the preperiod and sites 11 and 12 represented the postperiod.

3

Postperiod tons per acre-feet used to determine percentage is the average value of sites 11 and 12 of 0.256 ton per acre-ft.

4

Site 16 represented the preperiod and site 15 represented the postperiod.

in this analysis. Large fluctuations in the amount of flow entering and leaving the basin are indicated in table 6. Changing 

the flow regime can increase or decrease the salinity that is 

transported through a system. To more clearly understand the 

amount of salinity load change that occurred from the preperiod to the postperiod, the data need to be flow adjusted to 

compensate for as many seasonal and climatological effects as 

possible. 

Flow-Adjusted SalinityTrends

Flow-adjusted salinity trends in McElmo Creek basin 

were analyzed by using output from S-LOADEST and residual 

plots with a LOWESS noise-reduction algorithm. S-LOADEST load estimations for site 1 indicate a decrease in annual 

salinity load of 39,800 tons (32 percent) from water year 1978 

through water year 2006, which is an average decrease of 

1,370 tons per year for the 29-year period (fig. 10A, table 9). 

Annual salinity-load estimates for site 6 indicate a decrease of 

about 7,300 tons (36 percent) from water year 1982 through 

water year 2006, which is an average decrease of 292 tons per 

year for the 25-year period. Site 17 was selected to represent 

a background site that is not affected by the Dolores Project. 

Annual salinity-load estimations for site 17 indicate a decrease 

of about 8,600 tons (12 percent) from water year 1978 through 

water year 2006, which is an average of 297 tons per year for 

the 29-year period (fig. 10B, table 9). The trend in load at site 

17 was considered to be representative of a natural trend in the 

region due to factors that were outside the scope of this study. 

The natural trend estimated at site 17 may also be present at 

site 1 and site 6, reducing the actual trend associated with 

salinity-control efforts. Site 1 represents the results of salinity control measures within the basin and the salinity-load 

trend basinwide. A more detailed summary of salinity trend 

data estimated by S-LOADEST can be found in table 9 and 

Appendix 1.1–1.6.

Analysis of partial residual plots with a LOWESS 

curve and timeline of events indicates the changes in salinity 

concentration over time. Figures 10A and 11 show salinity 

concentration decreases occurring in the late 1980s and early 

1990s. Salinity data were analyzed to estimate the salinity 

load change during this period, which is also associated with 

salinity-control efforts in the basin. Using S-LOADEST to calculate loads at site 1 for water years 1989–2006, a decrease of 

approximately 21,700 tons (21 percent) was estimated, which 

is an average decrease of 1,210 tons per year for the 18-year 

period (table 10) and represents 55 percent of the total trend 

during the water years 1978–2006 period (table 9). Estimates 

for site 6 for water years 1990–2006 indicate a decrease in 

salinity load of about 4,000 tons (25 percent), which is an 
DOLORES

Dol

McElmo Creek

Navajo Wash

Yellow Jacket Canyon

Alkali Creek

Hartman Draw

Mud Creek

Simon Draw

Narraguinnep Canyon

Duncan

Ute Mountain

Main No. 2

McPhee

Trulson

Upper Arickere

Lower Arickere

Big Cork

Little Cork

Goodland

West 2

Towaoc Canal

Cortez

Towaoc Canal

Rocky Ford Ditch

Highline Ditch

Reservoir

Highline

Lone Pine Lateral

Preperiod canal

Alkali Creek subbasinHartman Draw subbasinMcElmo Creek 666 subbasinMud Creek subbasin

Crow Canyon subbasinGoodman Canyon subbasinNavajo Wash drainage

Postperiod canalStreamsColorado Division of Water Resources streamgaging siteBureau of Reclamation streamgaging siteU.S. Geological Survey streamgaging site

EXPLANATION

PrePost Postperiod

Orientation of flow

McElmo Creek watershed subbasins

Lone Pine Lateral

Dove Creek Canal

Garrett

Hermana Towaoc

West 1

East

May Lateral

U-Lateral

Moonlight Narraguinnep Reservoir

Totten

Res.

CORTEZ

COLORADO

UTAH

Summit Ditch

Camp Ditch

South Main 

Site 1 TonsPre 772,000Post 475,000

==

Site 6 TonsPre 30,600 Post 22,500

==

Site 3 TonsPre 52,700 Post 38,700

==

Sites 10,13,14 TonsPre 82,700 Post --

==

Site 11 TonsPre -- Post 7,900

==

Site 12 TonsPre -- Post 91,800

==

Site 15 TonsPre --Post 136,000

==

Site 16 TonsPre 77,100 Post --

==

Preperiod

Main No. 1

Figure 9. Pipe diagram comparing non-flow-adjusted salinity load in tons from water years 1978–1984 (preperiod) to salinity load in tons from w2000–2006 (postperiod) at selected sites in the McElmo Creek basin area, southwest Colorado.
Characterization of Hydrology and Salinity in McElmo Creek 25

1980 1985 1990 1995 2000 2005

-1,000

-500

0

500

1,000

1980 1985 1990 1995 2000 2005

-200

-100

0

100

Partial residuals, in milligrams per liter Partial residuals, in milligrams per liter 

A

B

EXPLANATION

Residual

LOWESS line

Natural Resources Conservation Service and Bureau of Reclamation 

on- and off-farm improvements begin

Reach 1 of Towaoc Canal

completed

Reach 2 of Towaoc Canal completed, all 

Bureau of Reclamation off-farm work completed

Water Year

Figure 10. Partial residual plot of salinity concentration with LOWESS curve at (A ) 

McElmo Creek near Colorado–Utah State line (site 1) and (B ) Dolores River at Dolores, 

Colorado (site 17).
26 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Table 9. Salinity-load estimates and flow-adjusted trend-analysis summary for water years 1978–2006.

[Numbers determined using S-LOADEST software (Insightful Corp., 2005a); 95th percent confidence interval, upper and lower limits; tons/day, tons 

per day; SEP, standard error of prediction; Annual load, in tons; Average change in annual load, in tons per year (tons/yr); ∆, change]

95-percent confidence 

interval

Water 

year

Daily mean 

salinity load

(tons/day)

Variance

(tons/day)

Lower 

limit

(tons/day)

Upper 

limit

(tons/day)

SEP

(tons/day)

Annual

load

(tons)

Percent 

change

Average 

change in 

annual load

(tons/yr)

Site 1

1978 344 35.4 331 356 6.39 125,000 --

2006 233 14.1 225 241 4.07 85,200 --

∆ annual load= –39,800 –32 –1,370

Site 6

1982 55.1 1.00 52.9 57.3 1.12 20,100 --

2006 35.0 0.70 33.3 36.7 0.90 12,800 --

∆ annual load= –7,300 –36 –292

Site 17

1978 192 29.9 181 204 5.82 70,000 --

2006 168 31.5 157 180 5.88 61,400 --

∆ annual load= –8,600 –12 –297

EXPLANATION

Residual

LOWESS line

Partial residuals, in milligrams per liter

1980 1985 1990 1995 2000 2005

-3,000

-2,000

-1,000

0

1,000

2,000

Natural Resources Conservation Service 

and Bureau of Reclamation on- and 

off-farm improvements begin Reach 1 of Towaoc Canal

completed

Reach 2 of Towaoc Canal completed, 

all Bureau of Reclamation off-farm 

work completed

Water Year

Figure 11. Partial residual plot of salinity concentrations with LOWESS curve of Mud Creek at State 

Highway 32 near Cortez, Colorado (site 6).
Characterization of Hydrology and Salinity in McElmo Creek 27

Table 10. Salinity load estimates and flow-adjusted trend analysis summary for water years 1989–2006.

[Numbers determined using S-LOADEST software (Insightful Corp., 2005a); 95th percent confidence interval, upper and lower limits; tons/day, 

tons per day; SEP, standard error of prediction; Annual load, in tons; Average change in annual load, in tons per year (tons/yr); ∆, change]

95-percent confidence 

interval

Water 

year

Daily mean 

salinity load

(tons/day)

Variance

(tons/day)

Lower 

limit

(tons/day)

Upper 

limit

(tons/day)

SEP

(tons/day)

Annual 

load

(tons)

Percent 

change

Average 

change in 

annual load

(tons/yr)

Site 1

1989 280 7.44 273 286 3.33 102,000 --

2006 220 12.7 213 228 3.87 80,300 --

∆ annual load= –21,700 –21 –1,210

Site 6

1990 43.6 0.40 42.2 45.1 0.76 15,900 --

2006 32.6 0.53 31.1 34.2 0.79 11,900 --

∆ annual load= –4,000 –25 –235

Site 17

1989 146 4.11 141 150 2.49 53,100 --

2006 134 8.81 128 141 3.25 49,000 --

∆ annual load= –4,100 –8.7 –228

average of 235 tons per year, for the 17-year period from 

1990–2006 (table 10) and represents 55 percent of the total 

salinity load reduction for the water years 1982–2006 period 

(table 9). A detailed summary of load estimations for water 

years 1989–2006 is provided in table 10.

The decrease in salinity load at site 1 was compared to 

estimations of salinity load reduction from salinity control 

by the USBR and Natural Resources Conservation Service 

(NRCS). The results showed a larger decrease in salinity than 

was anticipated by the USBR or the NRCS in their salinityreduction estimates. Estimates from the Dolores Project 

Report included a reduction of 32,000 tons per year from 

salinity control modifications associated with the Dolores 

Project (Bureau of Reclamation, 1988). The NRCS estimated 

an additional reduction of 18,000 tons per year for its contributions to on-farm and off-farm improvements (Frank Riggle, 

oral commun., 2009). Of this additional reduction, NRCS 

records estimate that 13,000 tons per year are attributed to 

their on-farm improvement, including sprinkler systems and 

on-farm piping projects, and an estimated 5,000 tons per year 

are attributed by the NRCS to off-farm improvements, including improvements to canals and the off-farm delivery system 

(Frank Riggle, oral commun., 2009) (table 11). The total 

estimated salinity-load reduction claimed by both the USBR 

and the NRCS is approximately 50,000 tons per year from 

water years 1978 to 2006 (table 11). The reductions offset the 

estimated increase of approximately 50,650 tons per year associated with the Dolores Project Plan (Bureau of Reclamation, 

1988), resulting in approximately no net change (650 tons) in 

annual salinity load. 

Analysis of the water-quality data for site 1 indicates 

that a large part of the downward trend in salinity load that 

occurred was not claimed by either the NRCS or the USBR. 

An additional reduction in salinity load of 39,150 tons is present in the salinity load trend for water years 1978–2006 at site 

1 (tables 9 and 11). 

Trend Analysis in the Mud Creek Subbasin

The Mud Creek subbasin comprises 22,051 acres of the 

McElmo Creek basin and contains 3,425 acres of irrigated 

lands watered from the Towaoc Canal (fig. 3). Initial analysis 

of the Mud Creek subbasin by the USGS indicated it might be 

a good candidate for determining on-farm and off-farm loads 

from a Mancos Shale setting because the subbasin contains 

predominantly Mancos Shale or Mancos Shale-derived soils 

(fig. 5). Analysis of the data and additional information supplied by the NRCS indicates that separation of on-farm and 

off-farm salinity loads was not possible. Salinity-control measures implemented by the NRCS and the USBR coincide with 

each other, making estimation of on-farm or off-farm loads 

unfeasible with the dataset that is available. The Mud Creek 

subbasin was one of the first areas where salinity-control measures were implemented by the NRCS. Sprinkler systems for 

1,236 acres were improved during 1990–1994, and the NRCS 
28 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

estimated salt reduction of 1,609 tons per year from this work. 

Additional improvements included 36,979 feet of on- and 

off-farm canals and laterals that were lined or piped, resulting 

in a predicted reduction of 560 tons of salinity per year (Frank 

Riggle, oral commun., 2009). 

Data analysis of the Mud Creek subbasin indicates a 

downward trend of salinity concentration and load beginning 

shortly after NRCS and USBR improvements began on the 

delivery canals and water delivery systems in 1990. Reductions in salinity concentrations were seen after the first and 

second sections of the Towaoc Canal were lined by the USBR 

in 1992 and 1994 (fig. 11). Reductions from 1994 through 

2006 are assumed to be a result of NRCS improvements both 

on-farm and off-farm. Separation of on- and off-farm trends is 

difficult due to the timing of improvements. Estimated salinity 

loads from S-LOADEST calculations indicate an estimated 

reduction of 7,300 tons in the Mud Creek subbasin from water 

year 1982 through water year 2006 (table 9), which is an average of 292 tons per year. S-LOADEST estimations for water 

year 1990 through water year 2006 indicate a downward trend 

of 4,000 tons, which is an average of 235 tons per year and 

represents 55 percent of the water years 1982 to 2006 trend 

(table 10). 

Natural Salinity in McElmo Creek Basin

Based on historical accounts from the town of Cortez, 

Colo., it was determined that McElmo Creek was not a perennial stream and was probably ephemeral in nature. Historical 

records indicate that the city of Cortez was established in 

late 1886 to supply housing for the Montezuma Valley Water 

Supply Company employees who were hired to build tunnels, 

ditches, and laterals that were required to divert water from the 

Dolores River into Montezuma Valley. The water supply for 

the settlement was hauled from a spring 2 miles from town; 

later, a well, which eventually went dry, was dug in the center 

of town (City of Cortez, 2008). This indicated that McElmo 

Creek was not a reliable source of water year round and that 

base flow was not adequate to maintain flows in the creek. 

Historical flows in McElmo Creek were assumed to be 

a result of snowmelt and high-intensity rainstorms, similar 

to the flow regime of many ephemeral streams in semiarid to 

arid environments. Analysis of streamflow hydrographs of 

average precipitation years in the basin indicated that between 

30 and 40 percent of the annual flow in McElmo Creek is a 

result of high-intensity precipitation and snowmelt. Geologic 

conditions in the basin are conducive to rapid-runoff conditions that create spikes in streamflow. Hydrograph analysis 

on a year-by-year basis indicates that late-summer to earlyfall high-intensity monsoonal storms create streamflow that 

can peak rapidly at more than 1,000 ft3/s, then recede in a 

matter of days. Historically, these events would have dominated the streamflow regime. The intensity of such storms is 

documented in a June 10, 1911, event that resulted in a rapid 

increase in streamflow in McElmo Creek that completely 

washed away orchards and irrigation flumes and laterals and 

incised a new channel within McElmo Canyon (City of 

Cortez, 2008). 

Because on- and off-farm salinity loads could not be 

determined separately in the Mud Creek subbasin, an alternative approach was taken to estimate the natural salinity 

load for McElmo Creek basin. The most concentrated salts 

are typically in areas where marine sedimentary deposits are 

present because dissolved solids precipitated to form salts as 

the ancient marine bodies evaporated. In order to determine 

a natural salinity load from McElmo Creek basin, all influences of irrigation, industry, and municipalities were removed, 

resulting in a historical, natural setting herein referred to as the 

historic period. 

The flow regime for the historic period in McElmo 

Creek was estimated using a water-budget analysis of the 

basin. Evapotranspiration (ET) rates for the irrigated land and 

the nonirrigated land were both estimated using the NRCS 

Table 11. Summary of salinity-load reductions and trend analysis at site 1 for water years 1978–2006.

[Unclaimed trend determined using S-LOADEST software (Insightful Corp., 2005a); USBR, Bureau of Reclamation; NRCS, Natural 

Resources Conservation Service; --, no data]

Estimated 

increase, tons

Actual 

increase, tons

Estimated 

decrease, tons

Claimed 

decrease, tons

Unclaimed 

trend, tons

Dolores Project Report 50,650 Unknown -- -- --

USBR off-farm -- -- –32,000 –32,000 --

NRCS on-farm -- -- -- –13,000 --

NRCS off-farm -- -- -- –5,000 --

Total decrease -- -- -- –50,000 --

Total trend -- -- -- 650 --

S-LOADEST trend -- -- -- -- –39,1501

1

 The unclaimed trend was calculated by subtracting the total trend in claimed decrease (650 tons) from the change in annual load 

at site 1 in table 9 (39,800 tons).
Summary 29

Blanney-Criddle method outlined in Ward and Trimble (2004). 

Crop coefficients for ET estimations were applied to a reference ET to estimate the volume of ET from each crop type. 

The crop coefficients used were (0.90) for alfalfa and (0.85) 

for grass pasture (Allen and others, 1998). The total water 

volume obtained from calculating the ET rate for irrigation 

was removed from the volume of water diverted into McElmo 

Creek basin from canals (983,000 acre-ft) for irrigation for 

the 7-year postperiod. Using this method it was estimated that 

86 percent of the diverted water for irrigation was consumed 

by crops within the basin. Therefore, an estimated 845,000 

acre-ft of water was consumed by crops and did not return to 

creeks as streamflow. The remaining 138,000 acre-ft that was 

not consumed was subtracted from the postperiod outflow of 

214,000 acre-ft. The remaining 76,200 acre-ft, or 10,900 

acre-ft per year for the 7-year postperiod, was assumed to 

represent a historical flow condition without the influence of 

diverted water for irrigation. The period from water year 2000 

to 2006 only represented 80 percent of the long-term precipitation average; therefore, the historical streamflow could be as 

much as 20 percent more than the estimated value of 76,200 

acre-ft. 

 Historical natural salinity loads were estimated using 

measured concentrations from natural surface-water sites, 

when possible, and wells. Sites were selected from sandstone and shale rock types in areas not affected by irrigation. Average TDS concentrations for each type of rock were 

used to estimate salinity loads. Most surface-water sites used 

to estimate a natural TDS concentration were springs. An 

average TDS value for water from sandstones in the basin 

was 350 mg/L, and the average for water from shale was 

4,000 mg/L. The historical flow of 10,900 acre-ft per year is 

equivalent to 15.1 cubic feet per second (ft3/s) and was entered 

into equation 1 along with the average TDS concentration 

for each rock type. The area of McElmo Creek basin that is 

underlain by sandstone is 129,000 acres, which is 56 percent 

of the basin and Mancos shale underlies 101,000 acres, which 

is 44 percent of the basin. Salinity loads for each rock type 

were estimated using equation 1 by taking the proportion of 

flow for each rock type, based on percentage of the basin, and 

the respective average TDS value (table 12). 

Natural salinity loads in McElmo Creek were estimated 

to be 29,100 tons per year for concentrations based on natural 

spring water (table 12). This is 43 percent of the 67,900 tons 

per year salinity load that was calculated for the postperiod 

(water years 2000–2006; table 8). The reported natural loads 

are considered estimates, and variation from year to year is 

expected. Salinity loads and concentrations are dependent 

on the locality and geologic conditions of the area affected 

by storms. Salt crusts on the surface of the soil in Mancos 

Shale areas can create an increase in concentration during 

rainstorms, which tapers off after the initial flushing. Reported 

salinity loads are an estimated average based on an estimated 

average concentration and may not accurately reflect these 

flushing events.

Summary

The Colorado Salinity Control Act, Public Law 93–320, 

June 24, 1974, was implemented to enhance and protect the 

quality of water in the Colorado River. Title II of the Colorado 

Salinity Control Act authorized the investigation and implementation of control measures for selected salinity-control 

units, including the McElmo Creek basin in Montezuma 

County, southwestern Colorado. Salinity is a concern in the 

McElmo Creek region of the Upper Colorado River Basin. 

Additional irrigation water storage, due to the development of 

McPhee Reservoir as part of the Dolores Project in the early 

1980s, lengthened the irrigation season, which potentially 

allowed for increases in salinity to McElmo Creek and its 

tributaries. 

Salinity-control projects such as irrigation and waterdelivery system improvements have been implemented in the 

McElmo Creek basin to reduce the overall salinity load to 

McElmo Creek. The Dolores Project, located in the McElmo 

Creek salinity-control unit, was developed to help support 

the agricultural industry, which farms about 62,000 acres of 

irrigated land in the area. To offset the increase of salinity due 

to the increased irrigation season and to maintain crop productivity, salinity-control features were completed as part of the 

Dolores Project. 

Interest in understanding water and salt budgets and also 

the effectiveness of salinity-control efforts since the construction of McPhee Reservoir has resulted in a network of waterquality monitoring sites, predominantly operated by the U.S. 

Table 12. Historical salinity load summary for McElmo Creek basin, southwest Colorado.

[USBR, Bureau of Reclamation; TDS, total dissolved solids; mg/L, milligram per liter; ft3/s, cubic feet 

per second; --, not applicable]

Rock type Area

(acres)

Average TDS 

concentration

(mg/L)

Average 

discharge

(ft3/s)

Average salinity 

load, (tons/year)

Sandstone 129,000 350 8.45 2,900

Mancos Shale 101,000 4,000 6.65 26,200

Total 230,000 -- 15.1 29,100
30 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Geological Survey (USGS) and Colorado Division of Water 

Resources (CDWR). Comparisons of the salinity loads exiting the basin during the preperiod (water years 1978–1984) 

and postperiod (water years 2000–2006) allows insight to 

the effectiveness of salinity-control features that were implemented and whether additional control is needed. 

Salinity-control features included in the Dolores Project 

were designed to help reduce and offset the salinity loading 

that resulted from increased water usage and longer irrigation seasons. According to revised estimates by the Bureau of 

Reclamation (USBR) for the 1977 Dolores Project Definite 

Plan Report, there would be 43,150 tons per year of salt would 

enter McElmo Creek from project lands and canals. Realignment of the Towaoc Canal would add another 7,500 tons per 

year, for a total of 50,650 tons/year, as a result of plan development. Salinity-control features, including the realignment of 

the Towaoc Canal, and project modifications were estimated to 

reduce the salinity load by 32,000 tons per year. Therefore, an 

estimated increase of 18,650 tons per year would be the resulting salinity load, not including any on-farm irrigation controls 

for reducing salinity loading. 

The streamflow regime of McElmo Creek is based 

largely on seasonal moisture patterns, augmented by regulated 

releases from Summit Reservoir and McPhee Reservoir. Variations in flow regime affect the salinity load; therefore, variations between daily salinity loads are common. 

Preperiod and postperiod salinity concentrations at 

selected sites in the McElmo Creek basin were compared. 

The median salinity concentration for site 1 (main basin 

outflow) decreased from 2,210 mg/L per day in the preperiod 

to 2,110 mg/L per day in the postperiod. The median salinity concentration for site 6 (tributary basin outflow) increased 

from 3,370 mg/L per day in the preperiod to 3,710 mg/L 

per day in the postperiod. Salinity concentrations typically 

increased from the preperiod to the postperiod at inflow sites. 

Salinity concentrations increased from 178 mg/L per day 

during the preperiod at site 16 to 227 mg/L per day during the 

postperiod at site 15. 

Non-flow-adjusted salinity trend analysis for the preperiod and postperiod indicates a change in salinity load 

after completion of the reservoir. Data analysis indicates the 

preperiod (water years 1978–1984) salinity load at site 1 was 

approximately 772,000 tons for the 7-year period. Navajo 

Wash (site 3) carried approximately 52,700 additional tons 

of salt out of the basin. The total salinity load exiting the 

McElmo Creek basin was about 824,700 tons during the preperiod. Inputs of salinity load to the watershed were calculated 

using data for sites 10, 13, and 14 combined; site 16; and site 

18. Salinity load entering the basin at these sites was estimated 

to be about 82,700 tons, 77,100 tons, and 6,990 tons, respectively, during the preperiod. Therefore, the total salinity load 

entering the basin at inflow sites was about 167,000 tons during the preperiod. The difference between the inflow salinity 

load and the outflow salinity load indicates the number of tons 

of salt picked up within the basin. Therefore, the amount of 

salinity picked up in McElmo Creek basin is 657,700 tons for 

the period 1978–1984.

 Analysis of postperiod data indicates the salinity load 

leaving the McElmo Creek basin at site 1 was 475,000 tons 

over the 7-year period from 2000 through 2006. An additional 

38,700 tons left the basin through Navajo Wash (site 3). The 

total salinity load exiting the basin during the postperiod was 

approximately 513,700 tons. Inflow sites for the postperiod 

were site 11, site 12, site 15, and site 18. The salinity loads 

entering the basin at each of these sites were 7,900, 91,800, 

136,000, and 4,560 tons, respectively, from 2000 to 2006. 

Total inflow of salinity load for the postperiod was 240,000 

tons. Therefore, the total amount of salt picked up within 

the basin during the postperiod was 273,700 tons, which is 

384,000 tons less than during the preperiod. 

Flow-adjusted salinity trends in McElmo Creek basin 

were analyzed by using output from S-LOADEST and residual 

plots with a LOWESS noise-reduction algorithm. S-LOADEST load estimations for site 1 indicate a decrease in salinity 

load by 39,800 tons from water year 1978 through water year 

2006, which is an average decrease of 1,370 tons per year for 

the 29-year period. Annual-load estimates for site 6 indicate 

a decrease of about 7,300 tons from water year 1982 through 

water year 2006, which is an average decrease of 292 tons per 

year for the 25-year period. Site 17 was selected to represent 

a background site that is not affected by the Dolores Project. 

Annual-load estimates for site 17 indicate a decrease of about 

8,600 tons from water year 1978 through water year 2006, 

which is an average decrease of 297 tons per year for the 

29-year period. The trend in load at site 17 was considered to 

represent the natural background trend in the region. 

The decrease in salinity trend at site 1 was compared to 

estimations of salinity load reduction from salinity control 

by the USBR and Natural Resources Conservation Service 

(NRCS). The results showed an excess decrease in salinity that 

was not claimed by the USBR or the NRCS in their salinity reduction estimates. Estimates from the Dolores Report 

included a reduction of 32,000 tons per year from salinitycontrol modifications associated with the Dolores Project. 

The NRCS estimated an additional reduction of 18,000 tons 

per year in 2006 for its contributions to on-farm and off-farm 

improvements. The total estimated salinity load reduction 

claimed by both the USBR and the NRCS is approximately 

50,000 tons per year from water year 1978 through water year 

2006. Analysis of the water-quality data at site 1 indicates 

that a large part of the downward trend in salinity load that 

occurred was not claimed by either the NRCS or the USBR. 

An additional reduction in salinity load of 39,150 tons per year 

is present in the salinity load trend at site 1. 

Calculation of the historical flow regime in McElmo 

Creek was done using a water-budget analysis of the basin. 

Using this method it was estimated that 86 percent of the 

diverted water for irrigation during the postperiod was 

consumed by crops within the basin; therefore, an estimated 

845,000 acre-ft of water is consumed by crops and did not 

return to the creek as streamflow. The remaining 
References Cited 31

76,000 acre-ft, or 10,900 acre-ft per year for the 7-year postperiod, was assumed to represent the historical flow condition 

without the influence of diverted water for irrigation. 

Average TDS concentrations for water from sandstone 

and shale in the area were used to estimate natural salinity 

loads. Most surface-water sites used to estimate a natural TDS 

concentration were springs. An average TDS value for water 

from sandstones in the basin was 350 mg/L, and the average 

value for water from shale was 4,000 mg/L. The historical 

flow of 10,900 acre-ft per year was converted to 15.1 cubic 

feet per second. Natural salinity loads in McElmo Creek were 

estimated to be 29,100 tons per year for concentrations based 

on natural spring water. This is 43 percent of the load that was 

calculated for the postperiod. Estimated natural loads may be 

widely variable with changes from year to year.

Acknowledgments

The authors of this report thank the following individuals. Thanks to Frank Riggle (Natural Resources Conservation Service) for supplying insight on agricultural practices 

and off-farm data; Stan Powers (Bureau of Reclamation) for 

providing data, insight, and knowledge throughout the report; 

Robert Wilson (formerly of the U.S. Geological Survey) for 

data collection; Cory Williams (U.S. Geological Survey) for 

support throughout the project and statistical insight; David 

Mau (U.S. Geological Survey) and Katharine Foster (U.S. 

Geological Survey) for providing technical reviews.

References Cited

Allen, R.G., Pereira, L.S., Raes, D., and Smith, M., 1998, 

Crop evapotranspiration—Guidelines for computing crop 

water coefficients: Rome, United Nations Food and Agriculture Organization Irrigation and Drainage Paper no. 56, 

300 p.

Bureau of Reclamation, 1988, Dolores Project Colorado: 

Supplement to definite plan report, Appendix B, Water 

Supply/Hydrosalinity, 131 p.

Bureau of Reclamation, 2003, Quality of water—Colorado 

River Basin: U.S. Department of the Interior Progress 

Report no. 21, 89 p.

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32 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

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CLIMATEDATA.html.
Appendix: S-LOADEST Salinity Load Output Tables 33

Appendix: S-LOADEST Salinity Load Output Tables

Table 13. S-LOADEST salinity load output1 for site 1 from water year 1978 to water year 2006.

[Flux, salinity load in tons per day; Variance, the variability of flux in tons per day; Lower 95, lower limit of the 95th percentile confidence interval; Upper 95, upper limit of the 95th percentile confidence interval; SEP, standard error of flux prediction, in tons per day; N, number of days; 

Load, salinity load in tons per month]

Date Flux Variance Lower 95 Upper 95 SEP N Load

Monthly loads

October 1977 403.9299 74.94589 379.8493 429.1214 12.57076 31 12,521.83

November 1977 390.7933 70.89384 367.2474 415.4375 12.29472 30 11,723.8

December 1977 325.6313 48.63212 306.2159 345.9425 10.13539 31 10,094.57

January 1978 312.1987 42.56686 293.8089 331.4264 9.59728 31 9,678.161

February 1978 425.8318 80.00632 399.5692 453.35 13.72116 28 11,923.29

March 1978 443.9319 90.8131 417.1231 471.9943 13.99927 30 13,317.96

April 1978 273.3916 32.52956 256.9087 290.644 8.606882 30 8,201.748

May 1978 229.2371 22.80389 215.6002 243.5019 7.118534 31 7,106.352

June 1978 268.7643 30.9509 252.8501 285.4075 8.306329 30 8,062.93

July 1978 287.3496 34.21494 270.5781 304.8779 8.750827 31 8,907.838

August 1978 351.6313 50.74442 331.1736 373.0091 10.6734 31 10,900.57

September 1978 420.9147 74.33005 396.0191 446.9494 12.99379 30 12,627.44

October 2005 281.5956 28.37686 265.679 298.2067 8.298664 32 9,011.058

November 2005 263.1313 30.65655 247.4459 279.5403 8.188206 30 7,893.939

December 2005 219.254 21.89784 206.2019 232.9074 6.813361 31 6,796.874

January 2006 210.2083 19.69931 197.7682 223.218 6.492987 31 6,516.458

February 2006 286.7165 38.6259 268.7972 305.5044 9.365183 28 8,028.062

March 2006 298.4566 42.13944 280.4759 317.276 9.388822 31 9,252.154

April 2006 180.5848 12.64101 169.9494 191.7046 5.550392 30 5,417.543

May 2006 157.561 8.552516 148.6265 166.8867 4.658646 31 4,884.391

June 2006 180.4993 10.66459 170.3798 191.0566 5.275177 30 5,414.978

July 2006 196.9584 12.17798 186.0809 208.2992 5.668438 31 6,105.71

August 2006 235.2241 18.49989 222.105 248.9078 6.838073 31 7,291.948

September 2006 287.6894 32.03042 270.9496 305.1821 8.733646 30 8,630.681

Annual loads

Water year 1978 343.5892 35.40149 331.238 356.2779 6.38797 364 125,066.5

Water year 2006 232.9065 14.10996 225.024 240.9915 4.073478 366 85,243.8

1

 Raw output data from S-LOADEST (Insightful Corp., 2005a), not adjusted to significant figures.
34 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Table 14. S-LOADEST salinity load output1 for site 1 from water year 1989 to water year 2006.

[Flux, salinity load in tons per day; Variance, the variability of flux in tons per day; Lower 95, lower limit of the 95th percentile confidence interval; Upper 95, upper limit of the 95th percentile confidence interval; SEP, standard error of flux prediction, in tons per day; N, 

number of days; Load, salinity load in tons per month] 

Date Flux Variance Lower 95 Upper 95 SEP N Load

Monthly loads

October 1988 376.3423 35.11645 356.3983 397.1001 10.38397 31 11,666.61

November 1988 332.9195 29.39587 314.9106 351.6792 9.380554 30 9,987.586

December 1988 253.4859 17.43116 239.8919 267.6416 7.07962 31 7,858.064

January 1989 259.3466 16.81278 245.6133 273.6398 7.150216 31 8,039.746

February 1989 346.5395 30.7163 327.1118 366.8078 10.12748 28 9,703.107

March 1989 311.406 24.15163 294.6305 328.8778 8.737295 31 9,653.586

April 1989 172.4141 6.433421 163.2659 181.9361 4.763193 30 5,172.424

May 1989 185.0094 7.053769 175.298 195.1131 5.055306 31 5,735.291

June 1989 208.4441 8.435192 197.6702 219.6464 5.606632 30 6,253.324

July 1989 233.8093 9.9547 221.8878 246.1981 6.202092 31 7,248.089

August 1989 301.4719 17.27481 286.1594 317.3822 7.965613 31 9,345.63

September 1989 382.7725 32.15915 362.5492 403.8182 10.52868 30 11,483.17

October 2005 295.6721 35.55362 278.482 313.6348 8.96845 31 9,165.836

November 2005 261.663 30.30842 246.0415 278.0063 8.155171 30 7,849.89

December 2005 199.3194 18.11428 187.4598 211.7251 6.190786 31 6,178.902

January 2006 203.9594 18.51091 191.8827 216.5894 6.303389 31 6,322.742

February 2006 272.5295 34.30058 255.5499 290.3296 8.873419 28 7,630.827

March 2006 244.9012 26.42326 230.2263 260.2568 7.661655 31 7,591.937

April 2006 135.5957 6.732651 127.7419 143.801 4.0971 30 4,067.87

May 2006 145.5025 7.182253 137.226 154.1424 4.315819 31 4,510.576

June 2006 163.9336 8.589132 154.7702 173.4922 4.776467 30 4,918.008

July 2006 183.8822 10.45142 173.7128 194.4855 5.299647 31 5,700.349

August 2006 237.0942 18.81622 223.8884 250.8676 6.883076 31 7,349.921

September 2006 301.0293 35.53325 283.4427 319.4102 9.17633 30 9,030.878

Annual loads

Water year 1989 279.8538 7.436798 273.3776 286.4427 3.332978 365 102,146.6

Water year 2006 220.0486 12.68671 212.5653 227.725 3.8674 365 80,317.74

1

 Raw output data from S-LOADEST (Insightful Corp., 2005a), not adjusted to significant figures.
Table 15. S-LOADEST salinity load output1

 for site 6 from water year 1982 to water year 2006.

[Flux, salinity load in tons per day; Variance, the variability of flux in tons per day; Lower 95, lower limit of the 95th percentile confidence interval; 

Upper 95, upper limit of the 95th percentile confidence interval; SEP, standard error of flux prediction, in tons per day; N, number of days; Load, 

salinity load in tons per month]

Date Flux Variance Lower 95 Upper 95 SEP N Load

Monthly loads

October 1981 60.75667 3.723138 55.6944 66.1515 2.668193 31 1,883.457

November 1981 32.35392 1.016975 29.69148 35.18897 1.40271 30 970.6177

December 1981 32.56373 1.039239 29.89949 35.39961 1.403379 31 1,009.476

January 1982 32.6166 1.022836 29.96268 35.44051 1.39769 31 1,011.115

February 1982 38.36821 1.384513 35.17827 41.76722 1.68121 28 1,074.31

March 1982 38.2338 1.341957 35.14141 41.52301 1.628282 31 1,185.248

April 1982 31.96629 0.941498 29.37342 34.72468 1.365392 30 958.9886

May 1982 68.62837 4.44333 62.98947 74.63223 2.970702 31 2,127.479

June 1982 81.65951 6.233324 75.03049 88.71209 3.490906 30 2,449.785

July 1982 80.3222 5.667588 73.94957 87.0922 3.353353 31 2,489.988

August 1982 81.23555 6.064487 74.71902 88.16322 3.430315 31 2,518.302

September 1982 80.95409 6.74875 74.20178 88.15047 3.559093 30 2,428.623

October 2005 38.54942 1.935221 35.11001 42.23191 1.817256 31 1,195.032

November 2005 20.52818 0.53376 18.71563 22.46742 0.957318 30 615.8455

December 2005 20.66208 0.511987 18.87795 22.56796 0.941544 31 640.5245

January 2006 20.69643 0.47258 18.95074 22.5583 0.920497 31 641.5893

February 2006 24.34688 0.600275 22.28626 26.54516 1.086693 28 681.7127

March 2006 24.26184 0.570828 22.27263 26.3795 1.047887 31 752.117

April 2006 20.28433 0.415058 18.60136 22.07736 0.886922 30 608.5299

May 2006 43.54839 1.955952 39.88996 47.44926 1.928807 31 1,350

June 2006 51.81562 2.924937 47.4408 56.48192 2.306913 30 1,554.469

July 2006 50.96475 2.918861 46.65539 55.56168 2.272508 31 1,579.907

August 2006 51.54253 3.274258 47.07118 56.32053 2.36007 31 1,597.818

September 2006 51.36311 3.629706 46.7245 56.33384 2.451976 30 1,540.893

Annual loads

Water year 1982 55.08874 0.998839 52.92079 57.32168 1.122728 365 20,107.39

Water year 2006 34.95463 0.659056 33.27217 36.69921 0.874305 365 12,758.44

1

 Raw output data from S-LOADEST (Insightful Corp., 2005a), not adjusted to significant figures.

Appendix: S-LOADEST Salinity Load Output Tables 35
36 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Table 16. S-LOADEST salinity load output1 for site 6 from water year 1990 to water year 2006.

[Flux, salinity load in tons per day; Variance, the variability of flux in tons per day; Lower 95, lower limit of the 95th percentile confidence 

interval; Upper 95, upper limit of the 95th percentile confidence interval; SEP, standard error of flux prediction, in tons per day; N, number of 

days; Load, salinity load in tons per month]

Date Flux Variance Lower 95 Upper 95 SEP N Load

Monthly loads

October 1989 51.39102 2.425028 47.18037 55.87337 2.218055 31 1,593.122

November 1989 24.06166 0.530443 22.09838 26.15107 1.034059 30 721.8499

December 1989 20.85432 0.413105 19.16499 22.65135 0.889556 31 646.484

January 1990 24.73121 0.517532 22.76701 26.81801 1.033616 31 766.6674

February 1990 26.47705 0.563978 24.35367 28.73435 1.11774 28 741.3574

March 1990 24.07694 0.466386 22.14692 26.12864 1.015942 31 746.3853

April 1990 23.56537 0.458874 21.70043 25.54628 0.981272 30 706.961

May 1990 58.42862 2.686399 53.8158 63.32757 2.426932 31 1,811.287

June 1990 67.27927 3.5265 62.04629 72.83175 2.7519 30 2,018.378

July 1990 66.53299 3.33994 61.43959 71.93216 2.677153 31 2,062.523

August 1990 65.99597 3.501298 60.8712 71.43309 2.694852 31 2,045.875

September 1990 68.90015 4.527589 63.24835 74.91709 2.977329 30 2,067.005

October 2005 38.40664 1.851289 34.99682 42.05618 1.801292 31 1,190.606

November 2005 17.9822 0.407868 16.38838 19.68788 0.841912 30 539.4661

December 2005 15.58554 0.304892 14.22475 17.04031 0.718424 31 483.1519

January 2006 18.48351 0.37256 16.91954 20.15191 0.824766 31 572.9887

February 2006 19.78868 0.396019 18.11514 21.5739 0.88253 28 554.0829

March 2006 17.99496 0.324046 16.47768 19.61318 0.800048 31 557.8437

April 2006 17.61246 0.322287 16.13839 19.18383 0.777063 30 528.3739

May 2006 43.66899 1.891992 40.02872 47.54852 1.918724 31 1,353.739

June 2006 50.28277 2.600492 46.09964 54.74025 2.204702 30 1,508.483

July 2006 49.72358 2.625428 45.58283 54.13619 2.182443 31 1,541.431

August 2006 49.32114 2.811982 45.11926 53.80568 2.216417 31 1,528.955

September 2006 51.49108 3.510562 46.8817 56.42738 2.435719 30 1,544.732

Annual loads

Water year 2006 43.63807 0.403237 42.16735 45.14638 0.759981 365 15,927.89

Water year 2006 32.6133 0.533049 31.08463 34.19687 0.793991 365 11,903.85

1

 Raw output data from S-LOADEST (Insightful Corp., 2005a), not adjusted to significant figures.
Appendix: S-LOADEST Salinity Load Output Tables 37

Table 17. S-LOADEST salinity load output1 for site 17 from water year 1978 to water year 2006.

[Flux, salinity load in tons per day; Variance, the variability of flux in tons per day; Lower 95, lower limit of the 95th percentile confidence 

interval; Upper 95, upper limit of the 95th percentile confidence interval; SEP, standard error of flux prediction, in tons per day; N, number 

of days; Load, salinity load in tons per month]

Date Flux Variance Lower 95 Upper 95 SEP N Load

Monthly loads

October 1977 40.26557 0.893911 37.67356 42.9872 1.355689 31 1,248.233

November 1977 46.94168 1.464416 43.74402 50.30904 1.674984 30 1,408.25

December 1977 115.8907 18.47344 106.0259 126.4184 5.203315 31 3,592.61

January 1978 376.9504 340.4961 337.903 419.2389 20.75588 31 11,685.46

February 1978 688.7389 1106.822 617.9387 765.3672 37.62166 28 19,284.69

March 1978 507.1954 298.5532 466.6068 550.3383 21.36426 31 15,723.06

April 1978 184.6291 18.55128 172.5368 197.3372 6.327431 30 5,538.874

May 1978 129.2057 16.21848 119.5045 139.4774 5.095981 31 4,005.376

June 1978 92.58728 11.89331 84.7361 100.964 4.140674 30 2,777.618

July 1978 64.73419 6.215376 59.15195 70.69682 2.94579 31 2,006.76

August 1978 50.31959 2.828318 46.37349 54.50872 2.075701 31 1,559.907

September 1978 40.27198 1.182284 37.46813 43.22808 1.469589 30 1,208.159

October 2005 35.28861 0.973668 32.80276 37.91112 1.303346 31 1,093.947

November 2005 41.13781 1.655952 38.01882 44.44215 1.638892 30 1,234.134

December 2005 101.5562 18.59494 92.07178 111.7447 5.019941 31 3,148.243

January 2006 330.3138 316.1743 293.4992 370.4372 19.63467 31 10,239.73

February 2006 603.5405 1016.827 537.1106 675.8602 35.40869 28 16,899.14

March 2006 444.4913 286.5365 406.2222 485.3639 20.1938 31 13,779.23

April 2006 161.8248 14.94537 151.1103 173.0911 5.608084 30 4,854.744

May 2006 113.2532 11.38591 104.9753 122.0042 4.344809 31 3,510.848

June 2006 81.15684 8.431544 74.45584 88.29379 3.530823 30 2,434.705

July 2006 56.7411 4.574324 51.91991 61.88569 2.542853 31 1,758.974

August 2006 44.1039 2.268065 40.59736 47.8296 1.845314 31 1,367.221

September 2006 35.29524 1.122912 32.68814 38.05304 1.368823 30 1,058.857

Annual loads

Water year 1978 191.8877 29.93164 180.7276 203.5495 5.822514 365 70,039.00

Water year 2006 168.1637 31.52441 156.9326 179.9787 5.879899 365 61,379.77

1

 Raw output data from S-LOADEST (Insightful Corp., 2005a), not adjusted to significant figures
38 Characterization of Hydrology and Salinity in the Dolores Project Area, McElmo Creek Region, Southwest Colorado

Table 18. S-LOADEST salinity load output1 for site 17 from water year 1989 to water year 2006.

[Flux, salinity load in tons per day; Variance, the variability of flux in tons per day; Lower 95, lower limit of the 95th percentile confidence interval; Upper 95, upper limit of the 95th percentile confidence interval; SEP, standard error of flux prediction, in tons per day; N, number of days; 

Load, salinity load in tons per month]

Date Flux Variance Lower 95 Upper 95 SEP N Load

Monthly loads

October 1988 63.67971 1.880746 59.72558 67.82421 2.066215 31 1,974.071

November 1988 50.02361 1.256015 46.8426 53.36166 1.663227 30 1,500.708

December 1988 40.83264 0.876981 38.23111 43.56286 1.360306 31 1,265.812

January 1989 38.96551 0.766695 36.50767 41.54365 1.284842 31 1,207.931

February 1989 42.43196 0.83544 39.72623 45.27165 1.414821 28 1,188.095

March 1989 101.0474 4.692727 94.57737 107.8392 3.383538 31 3,132.47

April 1989 271.0859 43.07007 252.9064 290.2144 9.51863 30 8,132.577

May 1989 496.9017 158.9303 463.5512 531.9945 17.4624 31 15,403.95

June 1989 326.3516 43.96826 306.1186 347.5572 10.57231 30 9,790.549

July 1989 126.4145 8.715746 118.2241 135.0172 4.284507 31 3,918.849

August 1989 98.88905 5.331119 92.53792 105.557 3.3216 31 3,065.561

September 1989 84.46043 3.32377 79.15704 90.02216 2.772042 30 2,533.813

October 2005 58.7427 2.82372 54.5415 63.17859 2.20368 31 1,821.024

November 2005 46.15409 1.928937 42.73938 49.76652 1.792936 30 1,384.623

December 2005 37.68167 1.332416 34.88203 40.64416 1.470177 31 1,168.132

January 2006 35.96156 1.149699 33.33347 38.73986 1.379405 31 1,114.808

February 2006 39.1619 1.212681 36.33113 42.1526 1.485303 28 1,096.533

March 2006 93.26071 6.725166 86.52773 100.3736 3.532661 31 2,891.082

April 2006 250.1963 56.20576 231.4746 270.0151 9.833431 30 7,505.888

May 2006 458.6128 199.1639 424.3654 494.8628 17.98708 31 14,217

June 2006 301.2127 60.02985 280.4987 323.037 10.85317 30 9,036.381

July 2006 116.6801 10.0423 108.5289 125.2754 4.27269 31 3,617.082

August 2006 91.27183 6.57373 84.82389 98.07535 3.380985 31 2,829.427

September 2006 77.95150 4.801134 72.38793 83.82519 2.918117 30 2,338.545

Annual loads

Water year 1989 145.5189 4.114463 140.7056 150.4529 2.486641 365 53,114.39

Water year 2006 134.3028 8.807987 128.0426 140.7865 3.251208 365 49,020.52

1

 Raw output data from S-LOADEST (Insightful Corp., 2005a), not adjusted to significant figures.
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Rodney J. Richards and Kenneth J. Leib—Characterization of Hydrology and Salinity in the Dolores Project Area, Southwest Colorado—SIR 2010–5218

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